Rich chocolate mixed with sweet cinnamon, baked to perfection.
Beat butter and granulated sugar at medium speed with an electric mixer until fluffy. Add egg, beating well. Combine flour and next 3 ingredients in a separate bowl; gradually add to butter mixture, beating just until blended. Stir in vanilla and chocolate.
Divide dough into 2 equal portions, and place on wax paper. Roll each portion into an 8-inch log. Place in freezer 30 minutes.
Brush logs with egg yolk. Divide turbinado sugar in half; roll each log in sugar, coating completely. Cut each log into 1/2-inch slices (about 15 slices), and place on parchment-lined baking sheets 1 inch apart.
Bake at 375 degrees for 12 to 14 minutes or until set. Remove parchment paper and cookies to wire racks; cool 2 to 3 minutes. Transfer cookies from parchment paper to wire racks to cool completely