Preheat your oven to 350°F (175°C) and line large baking sheets with parchment paper. In a medium-sized mixing bowl, whisk together the flour, cornstarch, baking soda, cream of tartar, and salt.

In a large bowl, cream the softened butter and granulated sugar together on medium-high speed for 3-4 minutes until light and fluffy.

Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla extract.

With the mixer on low speed, gradually add the dry ingredients to the wet mixture, mixing just until a soft dough forms.

Divide the cookie dough into five equal portions in smaller bowls. Add a tiny drop of each pastel gel food coloring to its respective bowl and mix thoroughly.

Roll the colored dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets. Lightly flatten each ball. Bake for 9-11 minutes, until edges are set but centers are slightly soft.

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

For the buttercream, beat the softened butter until creamy. Gradually add the powdered sugar, heavy cream, vanilla, and salt. Beat on medium speed for 2-3 minutes until fluffy.

Match the cooled cookies in pairs (by color or mix). Using a piping bag, pipe a swirl of buttercream on the bottom of one cookie. Press a second cookie on top to create a sandwich.