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 375º 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.