Preheat and prep the pan. Heat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving an overhang for easy lifting. Lightly grease the parchment to prevent sticking.

Whisk the dry ingredients. In a medium bowl, whisk flour, baking soda, and salt until combined. Set aside.

Mix the wet ingredients. In a large bowl, whisk melted, slightly cooled butter with brown sugar and granulated sugar until smooth and glossy, about 30 seconds. Add eggs and vanilla, whisking until fully incorporated.

Bring the dough together. Use a spatula to fold the dry ingredients into the wet just until no dry streaks remain. Don’t overmix. If the dough looks very loose, sprinkle in the extra 2 tablespoons flour.

Add the chocolate. Fold in M&M’s and chocolate chips. Reserve a small handful of M&M’s for the top.

Spread and top. Scrape the dough into the prepared pan and spread evenly to the corners. Scatter reserved M&M’s and sprinkles over the surface. Gently press them in so they stick.

Bake. Bake for 22 to 28 minutes, until the edges are golden and a toothpick inserted near the center comes out with a few moist crumbs (not wet batter). For chewier bars, pull closer to 22–24 minutes.

Finish and cool. If using, sprinkle a tiny pinch of flaky sea salt over the hot bars. Cool in the pan on a rack for at least 30 to 45 minutes to set.

Slice and serve. Lift out using the parchment, then cut into 16 to 24 bars. Serve slightly warm or at room temperature.