2¼ cups all purpose flour
½ Tsp table salt
½ Tsp baking soda
12 TB salted butter, melted
1 cup light brown sugar, packed
½ cup granulated sugar
1 large egg, room temp
1 large egg yolk, room temp
1 TB pure vanilla extract
1½ cups M&M's (holiday colors are great)
Preheat oven to 325F with a rack in lower middle position. Line a 9x13 baking pan with foil, with enough overhang on either side to make a sling (for ease of removal after baking.) Grease foil and set aside.
In a bowl, whisk together the flour, salt, and baking soda.
In another bowl, stir together the melted butter and both sugars. Stir in the egg and extra egg yolk. Stir in vanilla extract.
Using a rubber spatula, fold together the dry mixture into the wet mixture, just until incorporated. Do not overmix. Fold in 1 cup of the M&M's. Reserve the rest for pressing on top.
Using slightly wet fingers, press dough into an even layer in the greased and foiled pan, making sure to get an even thickness throughout. Press remaining M&M's on top of dough.
Bake 25-28 minutes or just until it's puffy and golden brown. It may seem a bit underbaked, but go ahead and take it out. It will set nicely upon cooling.
Let cool completely at room temp before removing with the foil sling. Remove entire dessert with the foil sling and place on a cutting board. Cut into squares. Leftovers will keep for days at room temp in an airtight container.