Cake Batter Bars

Cake batter is all the rage, didn't you know?
I've know this ever since I got hooked on cake batter ice cream but now there is so many things that you can make and buy that are cake battered and delicious.
I made Funfetti Cookie sandwiches last summer so I wasn't worried about the outcome of using the cake mix again, since everyone loved them.

I had a bit of an interesting time making these bars.
Everything was baking as usual and then I went to take them out of the oven and they had risen and were al the way to the brim of the pan! So I decided to just cut them into really small pieces to make up for how tall they were, simple solution, right? Well when I went back into the kitchen an hour later, the middle had sunk back down and the edges were still as tall as before! Just my luck. I brought them to work cut and hoped that no one would notice the difference.
I really don't know what would be the solution to my problem, any ideas?

I got the recipe from my favorite cooking blog, Lovin' From the Oven
The bars were still pretty amazing even with my mishap.
Jump on the cake batter wagon and take a bite.

Cake Batter Bars

1 1/2 cup funfetti cake mix
3/4 cup flour
1/4 tablespoon baking soda
3/4 cup unsalted butter, (melted & slightly cooled)
1 egg
1 egg yolk
2 teaspoons vanilla
1/2 cup sugar
1/2 cup brown sugar
5 heaping tablespoons sprinkles

Preheat oven to 350 degrees.
Grease a 8x8 pan.
Cream together the sugars, butter, vanilla, egg and egg yolk.
Gradually mix in the flour, cake mix, and baking soda until just mixed.
Stir in the sprinkles.
Spread batter evenly into greased pan.
Bake 30-35 minutes, until top is lightly browned and middle jiggles slightly.
Cool completely before cutting.