Preheat your oven to 325°F so your bars bake soft and chewy without over-browning. Line a 9x13-inch pan with parchment paper or lightly grease it if you prefer. I always leave a little overhang on the sides, it makes lifting the entire batch out so much easier later.

In a large bowl, whisk together the melted salted butter with the brown and granulated sugars until the mixture looks smooth and a little glossy.

Add the egg and extra yolk, then pour in the vanilla extract, whisking until everything is well combined. If your butter is still just a bit warm, that’s actually a bonus, it helps melt the sugars slightly, giving the bars a tender, fudgy texture.

Add the flour, baking soda, and a pinch of salt to the bowl. Lightly incorporate everything with a spatula, mixing only until the batter looks unified. You want the dough soft and tender.

Next, fold in the dark chocolate chips and most of the M&M’s, leaving a few aside to sprinkle on top. Press a few of the reserved M&M’s gently into the top of the dough, it adds a festive, colorful touch that makes the bars look extra special.

Spoon the dough into your prepared pan and spread it evenly. I like to use an offset spatula or the back of a spoon to smooth it out, but don’t press too hard, the bars should stay thick and soft in the center.

Bake for 22–25 minutes. The edges should be lightly golden, while the center stays soft, it will firm up as it cools. If you prefer extra fudgy bars, check around 20 minutes and remove them once the edges are set.

Let the bars cool in the pan for about 10 minutes before cutting. Run a knife around the edges first to help lift the whole batch out neatly. Enjoy!