The key to the whole thing is the pan. My husband bought it for me for Christmas. He got it at Big Lots for $5 apparently, but I am guessing you can get them online or in other stores.
It is silicone, there are two identical pan pieces, one for the top half, one for the bottom.
I preheated the oven as directed on the box cake mix. I prepared the batter according to the box directions. Then spray the pans thoroughly with Baker's Joy to ensure easy cake removal. Divide the batter evenly between the two pans. Mine have max fill lines inside them.
The instructions that came with the pans gave recommended bake time, I think it was about 30 minutes. You can do the toothpick test to check. Let them cool in the pans for a bit then cool completely on a rack.
Flip them back over and use a cake leveler to get a nice even surface on each. I used standard store-bought icing to put a layer across the cut surface of the bottom cake. Then carefully flip the other cake over on top of the first, being careful not to break the cake and line up the edges as best you can.
Next step is icing. I scooped more out of the same container as before and put it in a microwaveable bowl. I warmed it up for 10-15 seconds - enough to make it soft, but not too runny. (If you make it too runny, like I did the first time, add another spoonful of room temp icing and stir.) Pour the warm icing over the top of the donut/cake and let it drizzle down the sides a bit. Once you've got the icing good, top with chocolate sprinkles.
Serve to a delighted birthday boy (or whoever you feel is deserving of a delicious cake, maybe even yourself).
Of course, you can make any combination of cake flavor and icing and decorations you want. You could even hollow out a "trough" and put a filling in before putting the top cake on.
Enjoy! And let me know how it goes.