Here's the big tip on cut-up cakes: You MUST grease and flour your pan. Even though the box recipe says to grease the bottom only, I sprayed my whole pan with Pam for baking and the cake came out easy-peasy.
The cake won't raise as high if you grease the sides of the pan, since the batter can't cling to the sides as it bakes. That's actually a good thing here. Your cake will still be delicious, but it will be a little flatter and slightly more dense, which makes it easier to work with. Most importantly, it will pop right out of the pan.
The original recipe, which calls for limoncello liqueur and fresh mint (and is amazing!), comes from Cooking Light via my friend Heather. I didn't have mint on hand, and left out the liqueur in deference to the kids, though none of them ate it anyway. You can make this with any combination of berries.
Here's the variation I made today.
Summer Berry Medley
1 cup fresh raspberries
1 1/2 cups fresh blackberries
2 cups hulled fresh strawberries, quartered
1 cup fresh blueberries
1/4 cup sugar (start with half and adjust according to taste)
1 tablespoon grated lemon rind
Juice of one lemon
Toss all ingredients gently and let stand for 15 minutes. Serve over cubed cake (yellow, white, angel food or pound cake would all be delicious) with slightly sweetened real whipped cream.
It's even more delicious if you happen to have a five-year-old princess around to eat it with you.