Preheat oven to 350F. Prepare an 8 x 8 inch pan with parchment (or aluminum foil).
In a mixing bowl, beat together sugars, butter, vanilla, egg, and egg yolk until creamy and smooth. Stir in flour, cake mix, and baking soda until well combined. Mix in the sprinkles.
Spread dough evenly in prepared pan, smoothing with a spatula.
Bake for 30-35 minutes until top is lightly browned and middle still jiggles slightly. Allow to cool for 15-20 minutes before removing foil from pan and cutting into squares.