In the bowl of a stand mixer with the paddle attachment, or in a large bowl with a hand mixer, beat the softened butter and cream cheese on medium speed until smooth and creamy, about 2 minutes. Add the granulated sugar and beat on medium-high for another 3 minutes until light and fluffy.

Add the egg and vanilla extract to the creamed mixture. Beat on medium speed until just combined, scraping down the sides of the bowl as needed.

In a separate medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, and salt.

With the mixer on low speed, gradually add the dry ingredients to the wet ingredients. Mix only until just combined, being careful not to overmix. The dough will be very soft.

Divide the dough in half, flatten each into a 1-inch thick disk, and wrap tightly in plastic wrap. Refrigerate for at least 2 hours, or up to 2 days. This step is mandatory to prevent spreading.

Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper. On a lightly floured surface, roll one disk of dough out to about ¼-inch thickness. Use a 2-inch round cookie cutter to cut out circles.

Place the cookies 1-2 inches apart on the prepared baking sheets. Re-roll scraps as needed. If the dough becomes too warm, return it to the refrigerator for 10 minutes.

Bake for 9-11 minutes, until the edges are set and the centers are just barely firm. Do not let them turn golden brown. Let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

While the cookies cool, make the buttercream. In a large bowl, beat the softened butter until creamy. Sift in the powdered sugar, one cup at a time, mixing on low. Once incorporated, add the vanilla and a pinch of salt.

Add 2 tablespoons of heavy cream and beat on medium-high speed for 3-5 minutes until the buttercream is very light and fluffy. Add more cream, one tablespoon at a time, if it's too stiff.

Divide the buttercream into small bowls for each color. Add 1-2 drops of gel food coloring to each and stir until uniform.

Pair up the cooled cookies. Transfer buttercream to piping bags and pipe a generous swirl onto the flat side of one cookie. Gently press another cookie on top to create a sandwich. Repeat with all cookies.