Add the dry ingredients, mixing on low just until combined, scraping down the bowl a few times to make sure the dough mixes well.
Roll 1 ½ tablespoons of dough into a small ball.
Place on the prepared baking sheets, about 2 inches apart, and press it down until it is ¼ to ½-inch thick.
Continue with the rest of the dough.
Bake for 7-12 minutes, then immediately transfer the cookies to a wire rack to cool completely.
To make the frosting, beat the butter, peppermint extract, and vanilla together.
½ cup butter, 1 teaspoon peppermint extract, ½ teaspoon vanilla extract
Sift in the powdered sugar and cocoa powder and beat until well incorporated and smooth.
1 ½ cups powdered sugar, ¼ cup unsweetened cocoa powder
Add in heavy cream and mix until well-combined.
1 ½ tablespoons heavy cream
Once the cookies have cooled completely, frost each cookie and top with crushed peppermint candies.
¾ cup crushed peppermint candies or candy canes
Notes
All-purpose flour: It's important to sift the flour before measuring it. To measure it, lightly spoon into the measuring cup and level.
Cocoa powder: The process for measuring the cocoa powder is the same as for the flour. I haven't tried using Dutch-processed cocoa powder in this recipe.
Butter: I use salted butter. If you're using unsalted butter, increase the salt in the cookies to 1/2 teaspoon and add a couple of pinches of salt to the frosting.
Sour cream: I recommend using full-fat sour cream in this recipe.