Step 1

Melt the butter with the mint in a small saucepan over medium heat, swirling occasionally, until fragrant, 2 to 3 minutes. Remove from the heat and let the mixture steep for 30 minutes. Meanwhile, place the flour, cocoa powder, baking powder, and salt in a medium bowl and whisk to combine; set aside.

Step 2

Pour the butter through a fine-mesh strainer into the bowl of a stand mixer fitted with the paddle attachment. (Alternatively, strain into a large bowl and use an electric hand mixer.) Press the leaves to extract all of the butter from the leaves; discard the leaves.

Step 3

Add the brown sugar and beat on medium speed until light and fluffy, about 3 minutes. Add the egg and melted chocolate and beat until completely incorporated, scraping down the sides of the bowl as needed. Gradually fold in the flour mixture on low speed until just combined. Cover the bowl with plastic wrap and refrigerate until firm, about 1 hour.

Step 4

Once chilled, remove the dough from the refrigerator. Arrange 2 racks to divide the oven into thirds and heat to 350°F. Line 2 baking sheets with parchment paper or silicone baking mats; set aside.

Step 5

Place the granulated sugar and powdered sugar in separate small bowls. Scoop 1 tablespoon of the dough, form into a ball, and place on a plate. Repeat with the remaining dough.

Step 6

Roll each ball in granulated sugar and then in powdered sugar to coat. Place on the baking sheet 2 inches apart. Bake for 5 minutes. Rotate the baking sheets between racks and from front to back halfway through. Bake until the cookies are light golden-brown on the bottoms and around the edges, 5 to 7 minutes more.

Step 7

Place the baking sheets on wire racks and cool 3 minutes. Use a flat spatula to transfer the cookies to a wire rack to cool completely.