In a bowl beat together the butter, powdered sugar, egg yolk, and vanilla until combined.
In another bowl sift together the flour, cocoa powder, and salt.
Now pour the flour mixture into the butter and start mixing with your hands.When you form a disk wrap it nicely with plastic wrap and let it sit in the fridge for at least one hour.
When the dough is firm, roll it onto a floured surface to desired thickness.Cut the cookies and place on parchment paper sheet and bake for 12 minutes at 180 °C or 350 °F.
Let the cookies to cool completely and make the frosting.
Beat the butter and vanilla, and slowly incorporate the powdered sugar.
Now pipe some frosting onto one cookie and sandwich it with another cookie.Repeat with the rest of the cookies.Serve them with a glass of milk of course.Enjoy!