Preheat the oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Melt the butter in a small saucepan over medium heat. Swirl occasionally as it foams until golden brown specks appear at the bottom and it smells nutty, about 3 to 5 minutes. Pour into a mixing bowl and cool for 5 to 8 minutes, until warm to the touch but not hot.
1/2 cup butter
Add the brown sugar to the warm browned butter and mix to combine. Add the egg, egg yolk, and vanilla extract. Beat on medium-high speed with a mixer or whisk for 3 full minutes, until pale, thick, and glossy.
1 cup light brown sugar, 1 large egg, 1 large egg yolk, 2 tsp vanilla extract
Fold in the flour, baking soda, and salt with a rubber spatula just until the flour streaks disappear. Do not overmix.
Fold in the M&M's and chocolate chips. Spread the dough evenly into the prepared pan and press a few extra M&M's into the top.
1/2 cup Christmas M&M's, 1/2 cup semi-sweet chocolate chips
Bake at 350°F for 23 to 26 minutes, until the edges are set and golden brown, the top is shiny and crackled, and a toothpick inserted an inch from the center comes out with moist crumbs.
Cool completely in the pan for at least 1.5 to 2 hours, then lift out with the parchment and slice into 16 squares.
Notes
Don't slice them early. Warm blondies look underbaked even when they're done, so let them cool completely. In a hurry? Finish them briefly in the fridge.Go by the visual cues, not the clock. Ovens vary, so look for set golden edges, a crackled top, and moist crumbs on the toothpick.Storage: Keep in an airtight container for up to 5 days at room temperature, 1 week in the fridge, or 2 months in the freezer.