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. Avoid overmixing.
05 - Divide dough in half, flatten into discs, wrap in plastic, and refrigerate for at least 30 minutes.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - On a lightly floured surface, roll one dough disc to ¼ inch thickness. Cut rounds with a 2-inch cookie cutter and place 1 inch apart on prepared baking sheets.
08 - Bake for 8–10 minutes until edges just begin to turn golden. Cool on sheets for 2 minutes, then transfer to cooling racks.
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 a generous layer of buttercream onto the flat side of half the cookies. Top with remaining cookies to create sandwiches.
12 - Allow assembled cookies to set for 20 minutes before serving to ensure filling holds properly.