Prep your pan and oven: Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting, and lightly spray the sides.

Cream the butter and sugars: In a large bowl, beat softened butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes. This adds air and helps create a soft, chewy texture.

Add eggs and vanilla: Beat in the eggs one at a time, then mix in vanilla. Scrape the bowl so everything blends evenly.

Combine dry ingredients: In a separate bowl, whisk flour, baking soda, baking powder, and salt. Add to the wet mixture in two additions, mixing on low just until combined. Don’t overmix.

Fold in the mix-ins: Stir in chocolate chips, half the Halloween M&M’s, and mini chips if using. Keep the remaining M&M’s for the top.

Press and top: Spread the dough evenly into the prepared pan. Press the remaining M&M’s over the surface and sprinkle on Halloween sprinkles for color.

Bake: Bake 22–28 minutes, until the edges are golden and the center looks set but slightly soft. A toothpick should come out with a few moist crumbs, not wet batter.

Cool slightly: Let the pan cool on a rack for at least 20–30 minutes. This helps the bars firm up for clean slicing.

Decorate: Melt white chocolate (and orange melts, if using) in short bursts in the microwave, stirring between each. Drizzle over the bars in zigzags. Gently press candy eyeballs onto the drizzle so they stick. Add more sprinkles if you like.

Slice and serve: Lift the slab out using the parchment, transfer to a cutting board, and cut into squares or rectangles. For clean edges, use a sharp knife and wipe between cuts.