Certainly, one could use orange to great effect with a dark chocolate glaze, but I had lemons on hand, so that was what I made.
Lemon Filling:
1 cup sugar
4 tablespoons cornstarch
1/2 teaspoon salt
1 cup water
2 tablespoons+ (depending on desired tartness) lemon zest (concentrated culinary oils work quite well here-a few drops go a long way)
1/2 cup lemon juice (strained to remove seeds)
2 tablespoons unsalted butter
Heat in a pan over medium heat whisking constantly. Bring to a boil and then go an additional minute. Chill before spreading. It will seem a bit gelatinous when it is removed from the icebox, but a few stirs will smooth things out. Don't freak out if you discover a bowl of yellow blob-that's how it is intended to appear. The colder it is, the better it will hold up to being coated under the hot chocolate glaze in the final step.