Step 1

Preheat oven to 375ºF and line 2 baking sheets with parchment paper.

Step 2

In a stand mixer fitted with the paddle attachment, cream the butter and sugar together until creamy. Beat in the egg and egg white, mixing until combined.

Step 3

In a medium bowl, whisk the flour, salt, and baking powder together. In a glass measuring cup, combine the milk and vanilla. With the mixer on low speed, add a third of the flour mixture, followed by half of the milk mixture. Repeat these steps until you've used up all of the dry and wet ingredients. Mix until just combined.

Step 4

Divide batter in half. Dye one half red and the other half blue.

Step 5

Using a small cookie scoop or two spoons*, drop the red batter onto one of the prepared baking sheets, leaving about 2 inches of space between each mound. Drop the blue batter onto the other baking sheet in the same manner.

Step 6

Bake cake cookies until the tops are springy and just barely dry, about 7-10 minutes. Allow them to cool on the pan for 5 minutes before transferring to a wire rack to cool completely.

Step 7

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter on medium-high speed until fluffy. Scrape down the sides of the bowl, then beat in the vanilla and salt.

Step 8

With the mixer on low speed, slowly add the powdered sugar, a cup at a time, making sure to scrape down the sides of the bowl frequently. Once incorporated, add the milk/cream and beat on high for at least 3 minutes until light and fluffy. Transfer frosting to a pastry bag fitted with a large round tip.

Step 9

Match each red cookie with a blue cookie similar in size. Pipe a dollop of buttercream onto the bottom side of one cookie, then top with another cookie, pushing the two cookies together gently. Serve and enjoy!