Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, beat the softened butter and sugar until light and fluffy, about 2 minutes. Add the egg and vanilla, mixing until combined.

In another bowl, whisk together flour, baking soda, baking powder, and salt. Gradually mix into the wet mixture on low speed. The dough will be thick yet soft. Add milk to combine.

Divide the dough into five equal sections and use gel food coloring to tint each section a different pastel color. Knead the dye in until evenly distributed.

Roll about 1 tablespoon of dough into a ball and place it on the prepared baking sheet, spacing them 2 inches apart.

Bake for 8-10 minutes until edges are set but tops aren’t browned. Let cool for 5 minutes before transferring to a wire rack.

In a mixing bowl, beat the butter, then gradually add powdered sugar. Stir in heavy cream (or milk), vanilla extract, and salt until fluffy. Adjust the consistency if necessary.

Match cookies by size and spread a generous amount of filling onto the flat side of one cookie. Top with the second cookie and lightly press them together.

Allow the sandwiches to rest for about 10 minutes so the filling firms up a bit.