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

In a large mixing bowl, cream together 1 cup of softened unsalted butter and 1 cup of granulated sugar until light and fluffy, about 3-5 minutes.

Beat in 1 large egg and 2 teaspoons of pure vanilla extract until fully combined.

In a separate bowl, whisk together 2 ½ cups of all-purpose flour, 1 teaspoon of baking soda, 1 teaspoon of baking powder, and a pinch of salt.

Gradually add the dry mixture to the butter-sugar blend, stirring well. Pour in 2 tablespoons of milk and mix until a soft dough forms.

Divide the dough and add gel food coloring to create your desired pastel shades, then roll into tablespoon-sized balls.

Place colored dough balls on a baking sheet and bake for 8-10 minutes, until edges are set.

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

For the buttercream filling, beat ½ cup of softened unsalted butter until creamy, then gradually add 2 cups of powdered sugar, 1 teaspoon of vanilla, a pinch of salt, and 1-2 tablespoons of milk until fluffy.

Assemble the sandwiches by spreading or piping the buttercream filling on one cookie's flat side and topping with another cookie.