Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, cream the softened butter, brown sugar, and granulated sugar until light and fluffy (2-3 minutes).

Add the egg and vanilla extract, mixing until fully combined and smooth.

Gradually mix in the flour, baking soda, and salt until just incorporated.

Fold in the chocolate chips and Halloween M&Ms, reserving some for decoration.

Spread the dough evenly in the prepared baking pan and press the reserved candies and candy eyeballs into the top.

Bake for 20-25 minutes until the edges are golden brown and the center is slightly soft.

Allow to cool completely in the pan on a wire rack before slicing into squares.