Prep the pan. Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the long sides. Lightly grease the parchment. Preheat your oven to 350°F (175°C).
Melt the butter. Melt the butter and let it cool for 5–7 minutes until just warm, not hot. This helps achieve a glossy, chewy crumb instead of a greasy one.
Whisk the sugars and butter. In a large bowl, whisk the melted butter with brown sugar and granulated sugar until smooth and slightly thick, about 30–45 seconds.
Add eggs and vanilla. Whisk in the eggs and vanilla until the mixture looks creamy and well combined. Don’t overmix.
Combine dry ingredients. In a separate bowl, whisk flour, baking soda, baking powder, and salt. Add this to the wet ingredients and fold with a spatula just until no dry streaks remain.
Fold in the chocolate. Gently stir in 3/4 cup of the pastel M&Ms and the chocolate chips. Reserve the remaining M&Ms for the top.
Spread in the pan. Scrape the dough into the prepared pan. Use an offset spatula or the back of a spoon to spread it into an even layer. Press the reserved M&Ms on top for a colorful finish.
Bake. Bake for 20–25 minutes, until the edges are set and lightly golden and the center no longer looks wet. A toothpick should come out with a few moist crumbs, not wet batter.
Cool and finish. Let the bars cool in the pan on a rack for at least 30–45 minutes. If you like a sweet-salty contrast, sprinkle a pinch of flaky sea salt on top while warm.
Slice and serve. Use the parchment overhang to lift the slab out. Slice into squares or rectangles with a sharp knife. Wipe the blade between cuts for clean edges.
