Add the egg, egg yolk, peppermint, and vanilla extract and mix.
Add the flour, cocoa powder, baking soda, and salt. Mix until just combined.
Add parchment paper to the baking sheet.
Make the cookie dough balls. Use 2 tbsp of the dough for bigger cookies or 1 tbsp of the dough for smaller cookies. Roll the dough balls in the remaining sugar and place them on the baking sheet. Leave about 2 inches in between each ball of dough.
Make a deep cavity in the middle of the cookie using your thumb or the back of a spoon.
Put the cookie sheet in the fridge and chill for 30 minutes.
Preheat the oven to 350 degrees.
Place the sheet of cookies on the center rack for 4 minutes, rotate the sheet, then bake for another 4 minutes. (if the center of the cookies is no longer indented, use the back of a measuring spoon to press lightly)
Cool the baked cookies on the pan for 10 minutes, then move to a wire rack to continue to cool.
To make the chocolate ganache, put the chocolate, cream, and vanilla extract into a microwave-safe dish and microwave in 15-second intervals until melted. Stir in between each time. Chill until the mixture thickens up.
Whip the ganache using a mixer until it is fluffy.
Spoon the mixture into the middle of each cooled cookie, then add the crushed peppermint.