01 -
Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper, leaving overhang on two sides, and lightly grease the parchment.
02 -
Combine the chopped white chocolate and butter in a heatproof bowl. Melt gently over a pan of barely simmering water or in short microwave bursts, stirring until smooth. Cool for 5 minutes.
03 -
Whisk the brown sugar and granulated sugar into the melted mixture. Add the eggs one at a time, then whisk in the vanilla until the batter looks glossy.
04 -
Add the flour and salt. Fold just until combined, then fold in 1/2 cup white chocolate chips. Do not overmix.
05 -
Spread the batter evenly in the prepared pan. Scatter 1/2 cup M&M's and the optional extra white chocolate chips over the top, pressing them in lightly.
06 -
Bake for 24-28 minutes, until the edges are set and a toothpick inserted near the center comes out with moist crumbs. Press the remaining 1/4 cup M&M's onto the warm top for brighter color.
07 -
Cool completely in the pan for at least 1 hour. Lift out with the parchment and cut into 16 squares with a sharp knife.