01 - In a medium bowl, whisk together the flour, baking powder, and salt until well combined. Set aside for later use.
02 - In a large mixing bowl, beat the butter and 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 incorporated and smooth.
04 - Gradually add the dry ingredients to the wet mixture, mixing just until the flour disappears and a uniform dough forms.
05 - Divide the dough in half, flatten into discs, wrap in plastic, and refrigerate for at least 30 minutes to firm.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
07 - On a lightly floured surface, roll one disc of dough to ¼ inch thickness. Cut out rounds with a 2-inch cookie cutter. Place cookies 1 inch apart on prepared sheets.
08 - Bake for 8–10 minutes until edges are just beginning to turn golden. Let cool on sheets for 2 minutes, then transfer to racks to cool completely.
09 - Beat the butter until creamy. Slowly add powdered sugar, milk, vanilla, and salt, beating until smooth and fluffy.
10 - Divide the buttercream into small bowls and tint each with a different pastel food coloring, mixing until uniform color is achieved.
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 at room temperature before serving to ensure filling holds properly.