I hold this truth to be self-evident: Cupcakes are better when filled. This, of course, is a lesson learned from Bake It In A Cake, which proves that there's no lack of material with which to fill them, ranging from cookie dough to dollhouse-sized pies to baklava.
But the ultimate cupcake-stuffer? Cupcakes themselves. By embedding miniature cupcakes—frosting and all—inside batter-filled standard-sized cups, you get a surprisingly delightful treat: The baby-cakes, sealed by the moisture of the cake batter, don't dry out, whereas the baked bit of frosting lightly spreads, browns, and adds a rich crunch, making for an overall taste and visual contrast which can't be beat.
Note: If you don't cover the mini cupcakes batter, the frosting will spread a bit and you might want to keep a cookie sheet under the muffin tin in case of drippage. Alternately, save a little batter and cover them completely before baking.