Preheat the oven to 350 degrees and spray a 9in cast iron with baking spray
Use an electric mixer to cream butter, sugar, and brown sugar together in a bowl.
Continue to beat with the electric mixer while adding in the egg and vanilla.
Set out a second bowl to sift the flour, baking soda, and salt together.
Slowly add ⅓ of the flour mixture at a time to the wet ingredients. Beat to mix well.
Repeat until all of the dry ingredients have been blended with the wet ingredients.
Add 1 ½ cups of the sprinkles to the cookie dough.
Transfer the cookie dough to the prepared skillet.
Press the top of the cookie dough into the skillet.
Top the cookie dough with the remaining ½ cup of the sprinkles.
Spread the sprinkles evenly on the top of the unbaked cookie.
Bake at 350 degrees for 35-45 minutes.
Set aside to cool completely.