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

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

In another bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined.

Divide the dough into portions based on how many different colors you want and add food coloring to each portion.

Scoop out tablespoon-sized portions of dough, roll them into balls, and place them on prepared baking sheets, flattening each slightly.

Bake in the oven for 10-12 minutes or until the edges are lightly golden.

Let the cookies cool for a few minutes before transferring them to wire racks to cool completely.

In a mixing bowl, beat the softened butter until smooth.

Gradually add the powdered sugar, mixing until smooth.

Mix in the vanilla extract and heavy cream or milk until creamy.

Add food coloring if desired for extra color.

Spread a generous amount of buttercream on the flat side of one cookie and top with another cookie, flat side down.

Repeat until all cookies are filled.

Arrange cookie sandwiches on a platter and enjoy or store them in an airtight container for up to a week.