Preheat the oven to 325°F. Line 2 cookie sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attach ment, add the butter, powdered sugar, peppermint extract, milk, and egg yolk. Mix on low until combined but chunks of butter remain visible.
Add the cake flour, cocoa powder, and sea salt, and mix on low until combined into a smooth dough; it should feel like play dough in your hands.
Using your hands, form the dough into 48 balls and place them close together on the prepared cookie sheets (they won’t spread while baking, so 24 should fit on each cookie sheet). Flatten each ball slightly. They should look like disks that are about 2 inches in diameter.
Bake for 20 to 22 minutes, or until set and dry. Let cool completely on the cookie sheets.
Sift powdered sugar generously over the tops of the cookies. Store in an airtight container for up to 2 weeks.
Notes
High Altitude — Bake at 325°F for 17 to 20 minutes or until set and dry.