01 - Whisk together the flour, baking powder, and salt in a medium bowl. Set aside for later use.
02 - Beat the butter and granulated sugar on medium speed until light and fluffy, approximately 2–3 minutes.
03 - Add the egg and vanilla extract to the butter mixture, mixing until thoroughly combined.
04 - Gradually incorporate the dry ingredients into the wet mixture, mixing just until the flour disappears. Avoid overmixing.
05 - Divide the dough in half, flatten into discs, wrap in plastic wrap, 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 disc to ¼ inch thickness. Cut rounds with a 2-inch cookie cutter. Place 1 inch apart on prepared baking sheets.
08 - Bake for 8–10 minutes until edges are barely golden. Cool on sheets for 2 minutes, then transfer to wire racks to cool completely.
09 - Beat the 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 with different pastel food coloring shades.
11 - Spread or pipe a generous layer of buttercream onto the flat side of half the cookies. Top with remaining cookies to form sandwiches.
12 - Allow assembled cookies to set for 20 minutes before serving to ensure filling holds properly.