In a large mixing bowl, cream together butter, granulated sugar, and brown sugar until light and fluffy, about 3-5 minutes. Add egg and vanilla extract, mixing until well combined.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually mix dry ingredients into wet mixture to form two distinct doughs.

Divide the dough into two equal portions, shape each into a disc, wrap in plastic wrap and refrigerate for at least 30 minutes.

Preheat oven to 350°F (175°C).

Remove dough from refrigeration, scoop out about 1 tablespoon of each dough, and roll into balls.

Place on a lined baking sheet, leaving space for expansion and create wells in the centers.

Bake the cookies for 10-12 minutes, until set but soft in the center. Allow to cool for 5 minutes before transferring to a wire rack.

In a small saucepan, heat heavy cream over medium heat until it simmers. Remove from heat and pour over chocolate chips, stirring until smooth.

Once cookies are cooled, fill the wells with ganache and decorate with toppings as desired.

Allow ganache to set for 15-20 minutes before serving.