Step 1

Whisk together flour, cocoa powder, baking powder, and salt in a medium bowl. Beat butter and sugar in a large mixer bowl at medium speed until creamy. Add egg, vanilla, and peppermint extract, beating to combine. Reduce speed to low and gradually add flour mixture; beat just until incorporated and dough comes together in large pieces. Divide dough in half and scrape out each half onto a piece of plastic wrap; shape into a disk and wrap well. Chill dough in the refrigerator for 30 minutes.

Step 2

Preheat oven to 350°F and line two baking sheets with parchment paper. Work with one section of dough at a time, and roll out dough to just shy of 1/4-inch thick on a lightly floured board. Use a floured 2-inch round cutter to cut out circles. Gather scraps to re-roll once, punching out 20 circles per disk. Place circles on baking sheets.

Step 3

Bake, rotating pans halfway through, for 9 to 10 minutes, until tops and edges are set. Cool for 10 minutes on baking sheets, then transfer to a wire rack to cool completely.

Step 4

Place chopped chocolate in a double boiler or in a heat-proof bowl over a saucepan of gently simmering water. Slowly melt the chocolate, stirring occasionally until glossy and smooth. Reduce heat to low so that coating will retain its consistency, and stir in peppermint extract. Line two baking sheets with parchment paper.

Step 5

Drop cookies (one at a time) into melted chocolate to completely coat. Use a fork to lift the cookies out of the chocolate, tapping the fork gently against the side of the bowl to allow excess chocolate to drain off. Place dipped cookies on prepared baking sheets and chill in the refrigerator for 30 minutes to set chocolate.

Step 6

Store thin mint cookies in an airtight container in the refrigerator, or freeze up to two months.