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

In a medium bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.

In a large bowl, beat the butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.

Add egg yolks, peppermint extract, and vanilla extract to the sugar mixture and blend until smooth.

Gradually fold the dry ingredients into the wet mixture until just combined.

Drop tablespoon-sized dollops of dough onto prepared baking sheets, spacing them 2 inches apart.

Bake for 10-12 minutes, until edges are set and middle is slightly under-baked. Cool on the sheets for 5 minutes before transferring to wire racks.

Beat the softened butter for the frosting until creamy, then gradually add the confectioner’s sugar and heavy cream until smooth.

Once cookies are completely cooled, pipe or spread the buttercream frosting on top.