Cream butter and sugar for about one minute.

Beat in the egg yolk, milk and vanilla. Mix well.

Add the flour, cocoa and salt all at once. Mix just until flour disappears and ingredients are incorporated.

Cover and refrigerate for about 1 hour.

Whisk egg white until foamy in a small bowl.

Using a small (I use a 1 inch) scoop or your hands, Shape dough into 1 inch balls.

Dip the cookie dough in egg white, then roll in nuts.

Place the cookie balls on greased baking sheet, 9-12 cookies per pan.

Make an indentation in center of each cookie with your thumb.

Bake at 350° for 10-12 minutes or until center is set, do not over cook.

When cookie is done, remove from oven and immediately place a chocolate kiss or a chocolate disc for melting in the center of the cookie. Press down the center with chocolate if the indentation has risen too much.

Let cool. Cookies may be frozen or covered on counter for up to a week.