In a large mixing bowl, combine butter and sugar. Cream until light and fluffy.
Beat in egg yolk, milk, vanilla, and peppermint extract.
In a large bowl, combine flour, cocoa, and salt. Blend well.
Add dry ingredients to the wet ingredients in the mixing bowl. Mix until well-blended.
Refrigerate for 1 hour or until dough easily forms into a ball.
Shape into balls slightly larger than 1-inch.
Pour the sprinkles into a small bowl, and roll each ball in the sprinkles 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.
Bake at 350 degrees for 10-12 minutes, or until set when you lightly touch an edge (carefully).
Remove from oven and allow to cool on pan for 1-2 minutes.
Press a peppermint patty into the middle of the cookie, into the indentation.
Remove cookies from pans with a spatula, and cool completely on wire racks.