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 a mixing bowl, using a hand mixer, beat the butter and sugar at medium-high speed until lighter in color and fluffy, 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.
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, about 1 hour.
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.
While the cookies cool, make the ganache.
Place the white chocolate and butter in a microwave-safe bowl. Set aside.
1 cup finely chopped white chocolate, 1 tablespoon unsalted butter
Pour the cream in a different microwave-safe bowl and heat to a simmer (or just until a few bubbles are visible).
¼ cup heavy cream
Carefully pour the hot cream over the white chocolate/butter mixture, stir then leave it alone 2 minutes.
After the chocolate has set for 2 minutes, stir until smooth.
If the white chocolate and butter don’t fully melt, place ½ of the mixture in a clean microwave-safe bowl and heat 10-30 seconds (stirring every 10 seconds) until the chocolate has melted and the mixture is smooth. Be careful the chocolate doesn’t get too hot – white chocolate is sensitive and can seize up if it gets overheated.
Add the hot mixture to the cooler mixture and stir until smooth.
Add the Crème de Menthe or flavoring and mix until smooth.
1 tablespoon Créme de Menthe liqueur
Once cookies have completely cooled, fill each with ½ teaspoon of Crème de Menthe Ganache. (The ganache should be soft. If necessary, reheat it in the microwave just until it has softened.)