01 - Whisk together flour, baking powder, and salt in a medium bowl until evenly distributed. Set aside for later use.
02 - Beat unsalted butter and granulated sugar on medium speed in a large mixing bowl until light and fluffy, approximately 2–3 minutes.
03 - Add egg and vanilla extract to butter mixture, mixing until thoroughly combined and smooth.
04 - Gradually incorporate dry ingredients into wet mixture, mixing just until flour disappears and dough forms. Avoid overmixing.
05 - Divide dough in half, flatten into discs, wrap tightly in plastic wrap, and refrigerate for minimum 30 minutes to firm.
06 - Preheat oven to 350°F. Line two baking sheets with parchment paper and set aside.
07 - On lightly floured surface, roll one dough disc to ¼ inch thickness. Cut rounds using 2-inch cookie cutter. Arrange 1 inch apart on prepared baking sheets.
08 - Bake for 8–10 minutes until edges begin turning golden. Cool on sheets for 2 minutes, then transfer to wire racks to cool completely.
09 - Beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt, continuing to beat until smooth and fluffy consistency achieved.
10 - Divide buttercream into small separate bowls. Tint each portion with different pastel gel food coloring, mixing until desired color reached.
11 - Spread or pipe generous layer of tinted buttercream onto flat side of half the cooled cookies. Top with remaining cookies, flat side down, to create sandwiches.
12 - Allow assembled cookies to set for 20 minutes at room temperature before serving.