Preheat the oven to 350°F and line a cookie sheet with parchment paper.

In a medium bowl, stir together the flour, baking soda, baking powder, and salt; set aside.

In a large bowl, beat 1 cup of softened butter and granulated sugar with a hand mixer until light and fluffy.

Add in the egg and vanilla extract and mix until fully combined.

Slowly add the dry ingredients to the wet mixture, about 1 cup at a time, beating on low speed.

Divide the dough into 4 separate bowls. Add 2-3 drops of different gel food coloring to each bowl and mix until the color is uniform.

Scoop 1 tablespoon of dough at a time, roll into a ball, and place on the cookie sheet 1.5 inches apart.

Bake for 7-8 minutes or until edges are just barely golden. Let cool for 5 minutes on the pan before moving to a wire rack.

To make the frosting: Beat 1 cup of butter and 2 cups of powdered sugar until smooth.

Add the remaining 2 cups of powdered sugar, vanilla, and a pinch of salt. Beat for 3 minutes until pale and fluffy.

Pipe the buttercream onto the bottom of one cooled cookie and sandwich with a matching or contrasting color cookie.

Store in an airtight container and enjoy!