Preheat oven to 325 degrees. Generously spray and line with parchment a 9x9" pan. This cake likes to stick, so don't be shy with your spray!
Using paddle attachment of stand mixer, cream together soft butter and sugar (or you can use an electric handheld mixer.)
Slowly add 1 whole egg, and 3 egg yolks to the creamed mixture. Make sure each addition is fully incorporated before adding the next.
Add flour and baking powder, ensuring there are no lumps and everything is evenly mixed. Pull mixture off of stand mixer and set aside.
Using whisk attachment, or handheld electric mixer, whip reserved egg whites to a medium peak. Fold into cake batter.
Pour batter into prepared pan. Use whole cherries to stud the top with fruit, leaving a bit of room between each cherry.
Bake in 325 oven, for about 30 minutes, checking and rotating after the first 20. Cake should be golden brown, but not dark, on top. Skewer should come out clean.
Remove from pan, and let cool. Enjoy!