Beat the butter and sugar until comes together.
Mix in the egg yolk, vanilla extract and milk.
Gradually add the flour, cocoa powder and salt, 1/2 cup at a time.
Measure a tablespoon of dough, roll it then line it over a baking pan lined with parchment paper.
Use the back of a measuring spoon to dent the center of the cookies, and refrigerate for one hour.
Bake at 350F (180C) for 10-14 minutes.
Remove from the oven, then use the back of a spoon again to press down the puffed center. Allow to cook completely.
Melt the chocolate, mix in the corn syrup, then gradually mix in the room-temperature heavy cream.
Fill the center of each cookie with the chocolate ganache, and sprinkle with cookies.