Preheat the oven to 350°.
Using a stand mixer, cream together the butter and sugars until light in color and fluffy in texture.
Add in the eggs, corn syrup and vanilla. Mix until thoroughly combined.
Next, add the flour, cocoa, cornstarch baking soda and salt. Mix until your chocolate cookie dough forms. The dough will be soft.
Set the dough aside and make the peppermint filling.
Make the filling by combining all of the filling ingredients in a small bowl with an electric hand mixer until well combined and mixed together.
Divide the dough out into 12 equal portions, using a little less than 1/2 cup of dough in each cookie.
Roll the cookie dough into a ball and then use your thumb to create a pocket in the middle of the cookie dough.
Spoon approx 1 tbsp of the peppermint filling into each pocket and then close the cookie dough around the filling.
Slightly flatten the cookie so the dough, so it is like a thick hockey puck (about 2 inches thick, the cookies will spread).
Repeat with remaining cookie dough. Place 6 cookies per tray.
Bake at 350° for 15-17 minutes. The cookie should start to crack and crinkle, it will spread slightly and the centers of the cookie should no longer look wet before taking them out of the oven.
Allow the cookies to cool completely on the pan.
Once the cookies have cooled, melt the semi sweet chocolate chips in the microwave until completely melted. (microwave for 30 seconds at a time, stirring in between each 30 seconds until smooth)
Spoon the melted chocolate onto each cookie and spread over the top.
Immediately top with a York peppermint patty, that has been cut in half.
Place the cookies in the fridge for at least 30 minutes before serving. Serve Chilled.