Preheat the oven to 350 degrees F and line a baking sheet with parchment paper or a baking mat.

In a medium mixing bowl, add the softened butter, granulated sugar, and light brown sugar.

Beat until the mixture is light and fluffy, about 3 minutes.

Add eggs and vanilla; beat until very light and fluffy, about 1 minute.

In a separate bowl, whisk together flour, cocoa powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing well after each addition.

Stir in 1.5 cups of Halloween M&M candies (reserve the rest for topping).

Refrigerate the dough for 30 minutes to prevent spreading.

Drop heaping tablespoonfuls of dough onto the prepared baking sheet, about 2 inches apart.

Gently press additional M&M candies on top of each cookie for decoration.

Bake for 8-10 minutes until edges are set but centers are soft.

Allow cookies to cool on the baking sheet for 3 minutes, then transfer to wire racks to cool completely.