01 - Whisk together flour, baking powder, and salt in a medium bowl until evenly combined. Set aside for later use.
02 - Beat butter and granulated sugar on medium speed until mixture appears light and fluffy, approximately 2-3 minutes.
03 - Incorporate egg and vanilla extract into the butter mixture, mixing until fully blended and smooth.
04 - Gradually add dry ingredients to the wet mixture, mixing only until dough just comes together. Avoid overmixing.
05 - Divide dough into two equal portions, flatten each into discs, wrap tightly in plastic wrap, and refrigerate for minimum 30 minutes.
06 - Set oven temperature 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, spacing 1 inch apart on prepared sheets.
08 - Bake for 8-10 minutes until edges begin showing slight golden color. Cool on sheets 2 minutes before transferring to racks to cool completely.
09 - Beat butter until creamy consistency. Gradually incorporate powdered sugar, milk, vanilla, and salt, continuing to beat until mixture achieves smooth, fluffy texture.
10 - Divide prepared buttercream among small bowls. Add different pastel gel food coloring to each bowl, mixing until desired pastel shades are achieved.
11 - Spread or pipe generous layer of colored buttercream onto flat side of half the cooled cookies. Place remaining cookies on top to form sandwiches.
12 - Allow assembled cookie sandwiches to rest for 20 minutes, enabling filling to set slightly before serving.