Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a mixing bowl, cream the unsalted butter, granulated sugar, and brown sugar together for about 3 minutes until light and fluffy.

Add in the egg and vanilla extract, mixing until well incorporated.

Whisk together the dry ingredients in a separate bowl: flour, cocoa powder, cornstarch, salt, baking soda, and baking powder.

Gradually add the dry mixture to the wet ingredients until just combined.

Fold in the white chocolate chips and Halloween M&Ms gently.

Drop spoonfuls of the batter onto the baking sheets, spacing at least 2 inches apart.

Bake for approximately 12 minutes, or until puffed but soft in the centers.

Let the cupcakes cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.