Preheat oven to 350 degrees Fahrenheit and lightly grease a cast iron skillet.
In a medium-sized bowl, whisk together the flour, baking powder and salt. Set aside. In a separate bowl, beat the butter and sugar together on medium speed until light and fluffy, about two minutes. Add the egg and vanilla extract and beat on medium-high speed until combined, scraping down the bottom and the sides of the bowl if necessary.
Add the dry ingredients to the wet ingredients and mix on low speed until a dough forms. Add the sprinkles and fold them into the dough. Once fully incorporated, put the dough in the cast iron skillet and use a rubber spatula to make an even layer. In the center of the skillet, place the marshmallows.
Bake for 20-22 minutes for a softer cookie with a gooey center, and 22-25 minutes for a crisper cookie. Let cool and enjoy!