Preheat your oven to 350℉ (180℃) and prepare your baking pan with parchment paper and a light spray of nonstick cooking spray.

In a bowl, sift together the flour, baking soda, and salt. Set this mixture aside.

Cream together the unsalted butter, dark brown sugar, and granulated sugar until light and fluffy.

Incorporate the eggs and vanilla into the creamed mixture, mixing well until combined.

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

Fold in the Reese's Pieces and chocolate M&M's evenly into the dough.

Transfer the dough to the prepared baking pan and spread evenly.

Bake in the oven for 18-20 minutes, rotating half-way through until golden on the edges.

Immediately press assorted Halloween candy into the hot bars when they come out of the oven.

Cool completely in the pan before slicing into squares.