In a small bowl, whisk together cocoa and melted butter. Let cool slightly.

In the bowl of a stand mixer fitted with the paddle attachment, beat softened butter, brown sugar, and salt at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Beat in cocoa mixture. Add egg and vanilla, beating until well combined. Add flour, and beat until fully incorporated and a smooth dough forms. Shape dough into a disk, and wrap in plastic wrap. Refrigerate for at least 1 hour or up to overnight.

Preheat oven to 325°F (170°C). Line a baking sheet with parchment paper.

Roll dough between 2 pieces of parchment paper to ⅛-inch thickness. Freeze until firm, about 10 minutes.

Using a 1¾-inch round cutter, cut dough, rerolling scraps and cutting to use all dough, and place 1 inch apart on prepared pan. (Refreeze dough if it gets too soft to work with.)

Bake until a slight indentation is left when top is pressed with a finger, 8 to 10 minutes. Let cool completely on pan.

Spread or pipe 2 teaspoons (10 grams) Peppermint Filling onto flat side of half of cooled cookies. Place remaining cookies, flat side down, on filling. Using a fork, dip sandwich cookies in Chocolate Ganache, letting excess drip off, and place on a wire rack. Garnish with candies, if desired. Let stand until ganache is set, 15 to20 minutes. Store in an airtight container for up to 3 days.