In a medium bowl, whisk together flour, cocoa powder, and salt; set aside.
In a bowl fit with a mixer, cream butter and granulated sugar on high; about 5 minutes. Wipe down the sides of the bowl with a spatula.
Slowly beat in egg yolks, one at a time until incorporated.
Beat in peppermint extract. Beat in milk.
Gradually add dry ingredients into wet ingredients. Once ingredients are combined, turn up mixer until dough slightly comes together; no more than 1 minute.
Cover bowl and refrigerate for 30 minutes.
Preheat oven to 350 degrees F. Line baking sheet with parchment paper.
Scoop dough with a cookie scoop and roll dough into ball.
Gently press center of ball with 1/2 teaspoon spoon. Dip spoon into a bowl with granulated sugar to prevent dough sticking to spoon.
Bake in oven 13-15 minutes. Once out of the oven, find the indent in the center and this time gently press a 1/2 tablespoon spoon into the center to flatten it slightly. Allow cookies to cool then transfer to a cooling rack.
TOPPING and FILLING
Combine 3/4 cup semi-sweet chocolate chips, 3 tablespoons butter, and 1 tablespoon cornstarch into a glass microwave safe bowl. SET POWER LEVEL OF MICROWAVE TO POWDER LEVEL 4. This is important as anything hotter will burn the chocolate. Microwave on power level 4 for 5 minutes or until chocolate is silky smooth. Stir chocolate half way through cooking then again after cooking. Candy canes should be crushed with a rolling pin before beginning.
Add chocolate to the center of the cookie. Gently jiggle cookie to allow chocolate to smooth out.
Do half of the cookies. Then immediately top with crushed candy canes, mini marshmallows, and chocolate jimmies. Repeat with the next batch of cookies. Remaining chocolate may need to be reheated again with the same instructions.
Allow chocolate to set then store in an air-tight container.