Preheat the oven to 350°F (175°C). Line a 9x13-inch pan with parchment, leaving an overhang to lift the blondies out later. Grease the parchment lightly.

Melt the butter in a light-colored saucepan over medium heat, stirring or swirling frequently until the butter turns golden brown and has a nutty aroma.

Transfer the browned butter to a heatproof bowl and cool for 5–10 minutes.

Whisk the browned butter and brown sugar together until smooth and slightly glossy.

Add eggs one at a time, whisking each until incorporated. Stir in the vanilla.

In a separate bowl, whisk together the flour, baking powder, and salt.

Fold the dry ingredients into the wet mixture in two additions, mixing until just combined—do not overwork.

Fold in M&M's or chocolate chips.

Spread the batter evenly in the prepared pan, smoothing the top with a spatula.

Bake for 25–30 minutes, until the edges are set and golden. A toothpick inserted near the center should come out with a few moist crumbs, not wet batter.

Cool completely in the pan on a rack before lifting out and slicing into bars.