Preheat oven to 325 degrees. Grease a 12 inch cast iron skillet and set aside.
In a large bowl combine the brownie mix, vegetable oil, and eggs. Stir until well-blended, around 50 strokes. Pour into bottom of cast iron skillet.
In the bowl of a stand or handheld mixer, add the butter and sugars, then beat with a paddle attachment on medium until smooth and slightly fluffy, around 3 minutes. Add the egg and beat on high for 1 minute, then add the vanilla extract and beat on high until combined.
Whisk together the flour, cake mix, and baking soda, then add to the wet ingredients and beat on low until just combined. Do not overmix. Gently stir in the sprinkles, then divide the dough in half, saving one portion in an airtight container in the fridge for another use. Pull chunks of the dough apart and flatten into thick discs, then place them in the skillet with the brownie mix, pressing each piece down into the brownie mix to cover the edges only.
Place skillet in the oven and bake for 35-40 minutes, or until the middle is just barely set. The center should remain slightly fudgy, but shouldn’t visibly jiggle when shaken. Remove from oven and let sit until cool. Using the pastry bag, swirl vanilla frosting onto the skillet brookie in any pattern you’d like. Sprinkle with additional funfetti sprinkles and serve.