Preheat oven to 350º. Grease a cast iron skillet with vegetable shortening.
In a large bowl, add softened butter and sugars. Beat until fluffy.
Add egg and vanilla to the sugar mixture and beat until combined.
In a medium bowl, add flour, baking soda, and salt.
Gradually beat in dry ingredients to wet ingredients.
Fold in chocolate chips and rainbow sprinkles.
Press cookie dough into the bottom of the prepared cast iron skillet.
Bake 45-50 minutes or until cookie edges are golden brown and toothpick comes clean from the center.
Add Peeps to edge of the cookie. Place back in the oven for 5 minutes or until Peeps start to expand and get toasty.
Allow cookie to cool completely before serving.