Heat oven to 375°F.
Combine butter, sugar and egg yolk in bowl. Beat at medium speed, scraping bowl often, until creamy. Add flour and cocoa mix gradually, beating at low speed 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 7-9 minutes or until set. Cool completely.
Melt chocolate chips and shortening in 1-quart saucepan over low heat, stirring constantly, 3-5 minutes or until smooth. Place chocolate mixture in small resealable plastic food bag. Snip corner of bag. Drizzle chocolate over cookies.
We recommend Land O Lakes® Chocolate & Raspberry or Chocolate & Mint Cocoa Classics® Premium Hot Cocoa Mix.