Preheat oven to 400°F.

Prepare a baking sheet by spraying it with cooking spray.

In a large bowl, beat butter and sugar together.

Add eggs and vanilla. Beat again.

Add flour, baking powder, and salt. Beat until dough is uniform. Split the dough into three equal bowls.

Add one color of food gel to each bowl. The amount of food coloring gel used will depend on the type you are using.

Form 1 1⁄2 - inch balls of dough and place them on your prepared baking sheet.

Using a flat-bottom glass lightly spray with cooking spray, flatten dough balls until they are about 1⁄4 - 1⁄2 -inch thick.

Bake whoopie pies for 8-10 minutes or until lightly browned. Transfer cookies to a cooling rack.

To prepare frosting, in a medium mixing bowl, cream butter and powdered sugar together.

Add 2 tablespoons of milk and vanilla. Beat well. If it is too thick, add more milk. If it is too thin, add more powdered sugar.

Place sprinkles in a shallow bowl.

Make whoopie pies by filling two cookies with frosting.

Roll each whoopie pie in sprinkles to coat the edges.