Heat oven to 375 degrees. Combine butter, sugar and egg yolk in large mixing bowl. Beat at medium speed, scraping bowl often, until creamy. Reduce speed to low. Beat, gradually adding flour and hot cocoa mix and scraping bowl often, until well mixed. Shape dough into 1-inch balls. Place 1 inch apart onto ungreased cookie sheets. Flatten cookies to 1/4-inch thickness with bottom of buttered glass dipped in sugar. Bake for 7-9 minutes or until set. Drizzle white icing over cookies when cooled.