Gather all ingredients on a clean workspace. Grease a 9x13-inch baking pan and preheat your oven to 350°F (175°C).

Melt the unsalted butter in a medium saucepan over medium heat, stirring continuously for about 5-7 minutes until it turns a rich, golden brown.

Remove from heat and let cool for a few minutes. Combine the browned butter with brown sugar and granulated sugar in a mixing bowl, stirring until smooth.

Add eggs and vanilla extract to the butter and sugar mixture, mixing until fully integrated.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.

Gradually stir the dry ingredients into the wet mixture until just combined. Be careful not to overmix.

Fold in white chocolate chips and M&M’s if using.

Pour the batter into the greased baking pan and spread evenly.

Bake in the preheated oven for 25-30 minutes. A toothpick should come out clean or with a few moist crumbs.

Once baked, allow to cool in the pan for 15-20 minutes before transferring to a cutting board and cutting into squares.