In a medium bowl, mix together the dry ingredients (flour through salt).

In a large microwave safe bowl, heat the butter and chocolate chips at 50% power in 30 second increments until melted.

Add the remaining cookie ingredients (sugar through egg yolk).

Stir the dry mixture into the wet mixture and mix until well combined. The dough will be sticky and soft.

Pat the dough down onto a piece of plastic wrap, cover it, and place in the freezer for about 15 minutes. The dough should then be easy to handle and not too sticky.

Preheat oven to 350° F.

Roll into 3/4" balls and place 1" apart on a Silpat or piece of parchment paper.

Bake for 4 minutes. The cookies will appear to be undone in the middle but should not look raw.  They will continue baking even after you remove the cookies from the oven.

Let the cookies sit for a minute or two on the baking sheet and then remove to a wire rack to cool completely.

While the cookies are cooling, prepare the filling.

Beat the butter in a medium bowl until fluffy.

Gradually beat in the powdered sugar.

Add the peppermint extract and salt and beat.

To assemble the cookies, spread about 3/4 teaspoon filling onto the center of a cookie. Place another cookie on top and sandwich the cookies together.

Repeat with the remaining cookies.

Cover and store for up to 1 week in the refrigerator.