In a large mixing bowl, combine butter and sugar. Cream until light and fluffy.

Beat in egg yolk, milk, and vanilla.

In a large bowl, whisk together flour, cocoa, and salt.

Add dry ingredients to the wet ingredients in the mixing bowl, and beat until just combined.

Refrigerate for 1 hour or until dough easily forms into a ball.

Shape into 1-inch balls.

Roll each ball in the chopped hazelnuts until well-coated.

Place each ball 2 inches apart on greased baking sheets.

Press an indentation in the center of each dough ball using your thumb or the end of a wooden spoon.

Bake at 350 degrees for 10-12 minutes, or until set when you lightly touch an edge (carefully).

Remove cookies to wire racks to cool completely.

Prepare ganache.

In a small microwave-safe bowl, combine chocolate chips, heavy cream, and butter.

Microwave on high at 30-second intervals until chocolate is melted and the mixture is smooth.

Whisk in Kahlua.

Pipe or spoon into the center of each cookie.

Allow 10-15 minutes for the ganache to slightly set. Devour.