In a medium bowl, whisk together the flour, cocoa, and salt. Set aside.
1 cup all-purpose flour, ⅓ cup unsweetened cocoa powder, ½ teaspoon kosher salt
In another mixing bowl, using a hand mixer, beat the butter and sugar at medium-high speed until lighter in color and fluffy for about 2 minutes.
8 tablespoons unsalted butter, ⅔ cup granulated sugar
Add the egg yolk, milk, and vanilla and continue beating at medium-high speed until fully incorporated.Note: Egg and milk should be at room temperature. 1 egg yolk, 2 tablespoons whole milk, 1 teaspoon pure vanilla extract
Set speed to low, add the flour mixture all-at-once, and stir just until no flour is visible.
Cover the bowl with plastic wrap and place it in the refrigerator until firm, for about 1 hour. Note: Chilling the cookie dough before baking is essential. It helps the cookies hold their shape and prevents excessive spreading. While the cookie dough rests, line 2 baking sheets with parchment paper, set oven rack to middle position, and preheat oven to 350°F.
When the dough is ready, form it into ¾-inch balls.
Firmly roll each ball in the finely chopped Crème de Menthe chips, and place the balls, 2-inches apart, on the parchment-lined baking sheets. Press an indentation in the middle of each ball (about ¾ way through the ball) with the greased rounded end of a wooden spoon handle or ½-teaspoon measure. 10 ounces Crème de Menthe baking chips
Bake the cookies for 10-12 minutes or until set.
Remove cookies/tray from the oven and allow to cool for 1 minute. (If needed, carefully reshape the indentation on each cookie.)
After 1-minute, transfer the cookies to the cooling rack to cool completely before filling.