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

In a mixing bowl, beat the butter and granulated sugar until light and fluffy, about 2 minutes. Add the egg and vanilla extract, mixing until fully incorporated.

In a separate bowl, whisk together flour, baking soda, baking powder, and salt. Gradually add to the wet ingredients while mixing on low speed until thick but soft. Mix in the milk until combined.

Divide the dough into five portions and tint each with a different pastel gel food coloring. Knead until evenly colored.

Scoop about 1 tablespoon of dough for each cookie, roll into a ball, and place on the prepared baking sheet, 2 inches apart. Bake for 8–10 minutes until slightly cracked on top. Cool on the wire rack.

In a mixing bowl, beat the softened butter until smooth. Gradually add powdered sugar, followed by heavy cream (or milk), vanilla, and salt. Beat until fluffy.

Match cookie pairs by size, spread a generous amount of filling on the flat side of one cookie, and top with a second cookie.

Let sandwiches sit for about 10 minutes to firm the filling before serving.