Place rack in middle of oven and preheat oven to 350 degrees. Spray the bottom and sides of a 10-inch cast iron skillet with nonstick cooking spray. Set aside.

In a large bowl, combine butter, granulated sugar and brown sugar and beat with an electric mixer on medium-high until light and fluffy, about 3 minutes. Add the egg and extra egg yolk and vanilla and beat until well combined.

In a medium bowl, whisk together flour, baking soda and salt. Add half of the flour mixture to the wet ingredients and mix on low speed until combined. Add the remaining half of the flour mixture and continue to mix until combined. Fold in the sprinkles until well distributed.

Spread the dough into the prepared skillet, smoothing the top with a rubber spatula. Bake until the edges and center are set and lightly browned, about 25-30 minutes. Cool on a wire rack for 15-20 minutes. Decorate with vanilla icing or serve topped with vanilla ice cream.