People who bake frequently often have their foolproof recipes they count on to produce predictable results. Sure, I enjoy trying new things (geez, get your minds out of the gutter) but when it comes to something as important as a birthday cake, experience has taught me to stick with the familiar. My familiar is a rich, gold cake that goes out at night and does my bidding. It is often observed in the company of a butter/shortening decorator's icing.
I can see you making faces at the mention of shortening (you shouldn't do that as your face will stay that way you know). The shortening helps keep the icing spreadable, and it also keeps the finished cake from melting at room temperature. There are fancier butter creams that do not rely on shortening for this effect, and certainly many will taste better. Again, I opt for reliability, not butter cream that will curdle, and cause me worry trying to remedy it. Besides, it has an entire cup of butter in addition to the shortening, so it isn't like you're going to die of some sort of butter deficiency disease. Wait until you hear what goes into the cake.
When making cakes such as the one I made for Danny's Birthday that require cutting into shapes, it is important to have something solid that will cut cleanly. My son does not like pound cake (fool, I know) which would be my first choice for a cake construction material. My second is this rich cake that has butter, an ungodly number of egg yolks, and milk. It has a delicious, rich crumb, yet it is somewhat dense making it easy to cut in sharp lines. I always make the cake a day ahead of decorating so it can firm up, tightly wrapped in the fridge as cold cake cuts much better. Contrary to what every cookery book I own tells me, I find a serrated bread knife does an excellent job trimming and slicing this cake, provided you don't use too much of a sawing motion.
Something I've learned over the years is that extra cake eliminates the, "What if I screw-up?" anxiety. Bake more than you think you will need (but do it as separate batches as the recipe does not double well) and freeze any extra tightly wrapped in wax paper and cling film. It makes a wonderful trifle, emergency cake (what, you're too classy to have a cake emergency?") or gift in a hurry (defrost and decorate). Too much cake is rarely a problem (really, when was the last time you heard someone complain of too much home baked cake? If you spend time with people that complain about an abundance of cake, you obviously need new friends) but too little when you've already started decorating is a pain. Bake ahead, and screw-up all you like secure in the knowledge that you have, emergency cake.
Filling: I don't bother. If you feel compelled to fill an already rich, decorated cake, stick with something like jam. If you're making layers, then I suppose there's an obligation, but for a sheet cake cut into designs, I consider it madness to split the cake and fill it. For a layer cake don't go for a heavy custard filling as it may squish out the sides making decorating difficult. I really like jam-it stays where you put it. A layer of melted chocolate that will harden again is also interesting. Whatever you do, keep the filling thin. You're not going to listen, are you? No, I know you too well, and can see the weeping lemon curd. Don't say I didn't warn you.
For the icing:
1 cup butter
1 cup vegetable shortening
2 teaspoons flavouring
3 cups sifted icing sugar (possibly more or less)
Cream the butter and shortening until light. Beat in flavouring. Add the sugar a half cup at a time until you have an icing that is a spreadable consistency. If you use liquid food colour rather than gel, you may find it thins the icing a bit, so you'll need to experiment. If you should make the icing too thick, thin it with a small bit of cream added a tablespoon at a time. The icing will begin to dry out as the air hits it, so if you are affixing something like jimmies or pearl sugar, do so before the frosting sets to ensure it adheres well. It will not turn hard like royal icing, though it can easily be piped on for a decorative effect. I would use extra icing sugar to make it stiffer.
Make a crumb coat of thin icing on the top and sides of the cake (I start with the sides, but that's me). Return the cake to the fridge for at least 20 minutes for the icing to set. This is a step well worth doing as the finished decorating looks smooth, and neat. I like tidy cake. I also like slide rules, advanced maths, Linnaeus, and things that order the universe in a way that seems pleasant without turning into my mother and colour coding the linen cupboard, or ironing the dusting rags. Obviously my world is not organised around punctuation. It is a cake for heaven's sake, not some bloody curry you threw together from vegetables rotting in the bins, and a tin of coconut milk. It really ought to look like you made some sort of effort. So crumb coat, OK?
For the Cake:
This will make a 9x13 or two 9 inch layers
1/2 cup butter
1 2/3 cup granulated sugar
5 large egg yolks (yes, that is quite a bit. Freeze the whites for later use) well beaten
2 1/2 cups sifted cake flour (use the cake flour for this, not plain flour)
2 1/2 teaspoons baking powder
1 teaspoon salt
1 cup milk (I used 2 % because I had it, but anything will do)
2 teaspoons vanilla extract ( or any flavouring you like)
Preheat oven to 350 degrees F. with rack in centre position. Butter and flour cake pan(s). Cream together the softened butter and sugar until fluffy. Beat in the eggs. Sift the dry ingredients together and add alternating with the milk and extracts. Don't over beat, but you do want to make sure it is mixed thoroughly. Bake 25-30 minutes for layers, 35-40 for 9x13. Cake should test done when it passes toothpick test and begins to pull away from sides of the pan. Coll 10 minutes in pan on a rack, then remove from pan carefully and cool completely on rack.
Put it together:
I like to make room in the fridge for two large baking sheets. Do this ahead. Cover the sheets with wax paper to catch any drips of icing, crumbs, etc. As you will be working in stages (you are going to do a crumb coat, correct?) it is easier to know exactly where the cake is going between steps. Baking sheet in hand is no time to discover you have a dozen jars of chutney and relishes that must be moved before the cake can chill. You probably don't need that jar of Branston pickle from 2006, so use this as an opportunity to bin it. Sure, your spouse/partner will insist it is still good, but you need the room for cake. Branston is the devil anyway.
After the frosting has set, lightly drape a piece of wax paper over the cake (or a cake cover if doing layers that will fit) and let it firm up at least half a day before serving. It cuts best for serving cold, though it does tend to taste better when closer to room temperature-with the exception of summer heat. Shortening may be designed to withstand the flames of hell (or a summer in the American South) but butter gets unpleasant after an hour or so in high heat.
Remember, even if the cake gets a bit stale, you can cut off the icing and make a perfectly acceptable trifle from it. Toasted slices of cake drizzled with Golden Syrup make a rather nice breakfast as well. Do not waste cake (god, I think I shuddered even typing "waste" and "Cake" in the same sentence).