Preheat the oven to 350F then line an 8x8 inch baking pan with parchment paper.
Mix the butter and brown sugar together on medium speed until combined.
½ cup butter, 1 cup brown sugar
Add the egg, the egg yolk, and the vanilla extract and whip for at least 3 minutes. This process will create a beautiful crinkly top.
1 large egg, 1 egg yolk, 2 teaspoon vanilla extract
Mix in the flour and salt just until the flour disappears. You don't want to over-mix the brown butter blondie mix. Fold in the chocolate chips and the M&m's.
1 ½ cup AP flour, 1 teaspoon salt, ¾ cup m&m's, ½ cup chocolate chips
Spread the blondie mixture in the prepared 8X8 inch baking pan, and bake for 20 minutes.
Once baked, pull out the blondies from the oven and allow them to cool down fully before slicing into them. I let them cool for an hour or two depending on my kitchen's temperature. They should be at room temperature. This will allow the bars to be fully set. Slice into 16 equal squares and enjoy.