01 - Whisk together flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat butter and granulated sugar on medium speed until light and fluffy, approximately 2-3 minutes.
03 - Add egg and vanilla extract to the butter mixture, mixing until fully incorporated.
04 - Gradually add dry ingredients to the wet mixture, mixing just until dough forms and no dry streaks remain.
05 - Divide dough in half, flatten into discs, wrap in plastic wrap, and refrigerate for minimum 30 minutes.
06 - Preheat oven to 350°F and line two baking sheets with parchment paper.
07 - On floured surface, roll dough to ¼-inch thickness. Cut rounds with 2-inch cookie cutter and place 1 inch apart on prepared sheets.
08 - Bake for 8-10 minutes until edges begin turning golden. Cool on sheets 2 minutes, then transfer to racks to cool completely.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt, beating until smooth and fluffy.
10 - Divide buttercream into small bowls and tint each portion with different pastel food coloring.
11 - Spread or pipe buttercream onto flat side of half the cookies. Top with remaining cookies to create sandwiches.
12 - Allow assembled cookies to set for 20 minutes at room temperature before serving.