1. In a large bowl, beat together 1 cup unsalted butter, 3/4 cup brown sugar, and 1/2 cup granulated sugar until light and fluffy, about 2-3 minutes.

2. Add 2 large eggs one at a time, mixing well after each. Stir in 1 teaspoon vanilla extract.

3. In a separate bowl, whisk together 2 cups all-purpose flour, 3/4 cup Dutch-processed cocoa powder, 1 teaspoon baking soda, and 1/2 teaspoon salt.

4. Gradually add the dry ingredients to the butter mixture until just combined. Chill the dough for at least 1 hour.

5. Preheat oven to 350°F (175°C). Scoop tablespoon-sized dough balls and place 2 inches apart on parchment-lined baking sheets.

6. Bake for 9–10 minutes or until the edges are set. Cool completely on a wire rack.

7. For the filling, beat 1/2 cup unsalted butter until creamy. Add 1 1/2 cups powdered sugar, 1/2 teaspoon peppermint extract, and 1 tablespoon milk or cream. Beat until smooth.

8. Pipe or spread 1 tablespoon of the filling onto half the cookies. Top with another cookie to create sandwiches.

9. If desired, roll the edges of the filling in 1/4 cup crushed peppermint candies for decoration.