Combine sugar, 3/4 cup butter, egg yolk, milk, and vanilla until creamy. Add chocolate and mix well. Add flour and salt. Chill dough until firm. Place dough in cookie press and press onto cookie sheet. Bake at 375 for 9-12 minutes. While cookies are baking, prepare the rum glaze.

To make the glaze, combine all ingredients and stir well. The glaze should have a semi thick consistency, and should not run off a spoon. Spread on cooled cookie and sprinkle with colored sugar sprinkles.