Preheat the oven to 350°F (175°C).

In a mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract, mixing until well combined.

In another bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Grease a muffin tin and scoop about 1 tablespoon of cookie dough into each cup, pressing down to form a cup shape.

Bake in the preheated oven for 12-15 minutes or until the edges are set.

Allow to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.

Remove from heat and add the chopped chocolate, stirring until smooth and fully melted.

Stir in the vanilla extract and let the ganache cool slightly.

Once the cookie cups are cool, fill each cup with the chocolate ganache.

Top with Halloween-themed sprinkles and mini chocolate chips for decoration.

Allow the ganache to set for about 10 minutes before serving.