Cream together butter and sugars until light and fluffy.
Add in egg, egg yolk, peppermint and vanilla extracts and mix until thoroughly combined.
Add cocoa powder, flour, baking soda and salt. Mix just until combined.
Line baking sheet with parchment paper.
Make balls out of 2 tablespoons dough each. Roll balls in the remaining ½ cup sugar. and place on the baking sheet, about 2 inches apart. Make a deep cavity in each cookie, using your thumb or the back of a measuring spoon.
Chill cookies on baking sheet for 30 minutes.
Preheat oven to 350°.
Bake cookies on center rack of oven for 4 minutes, rotate baking sheet, then bake for an additional 4 minutes. If the indentations have lost their shape, push your thumb or measuring spoon in again to remake.
Cool in pan for about 10-min. then remove to wire rack to finish cooling.
Chocolate Ganache
Place chocolate, cream and vanilla extract into microwave-safe dish and microwave in 15-second increments, until melted. Stir with each 15-second increment. Chill until mixture thickens.
Either spoon or pipe ganache into each cookie and immediately sprinkle with crushed peppermint.
Notes
Be sure butter is at room temperature.Chill the unbaked cookies and the ganache for the listed time.