Preheat the oven to 350°F. Line a square 8x8 inch baking pan with parchment paper. Square baking pans can be tricky to line with parchment paper. I like to first spray the pan with nonstick cooking spray (or coconut oil spray). Then, lay a sheet of parchment paper over top of the baking pan. Using scissors, cut in diagonally from each corner until the cut is about 1 inch past the corner of the pan. After all corners have been cut, press down on the center of the parchment paper to fit the parchment into the pan. Tuck the flaps (the ones that look like triangles) behind so the parchment sits flat in the pan and along the edges. You can use small binder clips to clip the parchment to the pan along the top of the pan, if desired. I find the parchment sticks well enough to the pan with the nonstick spray, so this is not always necessary. Set the parchment-lined pan aside.

Melt the butter in a small skillet over low heat or in a microwave-safe bowl. Be sure to cover the top of the bowl with a paper towel or wax paper before microwaving to prevent the butter from popping and exploding all over the microwave. Alternately, melt butter over low heat in a small saucepan, removing once the butter has completely melted.

In a large bowl, add the brown sugar and pour the melted butter over top. Whisk until well blended; the mixture will resemble wet sand.

Add in the egg and vanilla and whisk to combine.

Add in the flour and salt, stirring with a wooden spoon until just blended and no dry flour spots appear.

Stir in the M&Ms and chocolate chips.

Scoop batter into prepared baking dish and press gently into the corners of the pan using a spatula or clean hands to form an even layer. If the dough is thinner in some areas and thicker in others, you will have uneven cookie bars. And, the thinner sections will bake faster than the thicker sections leaving you with overbaked bars in spots and underbaked bars in other spots. Sprinkle additional m&ms on top of the bars.

Bake in the preheated oven for 20-25 minutes until edges are golden brown

Cool bar cookies in the pan then lift out of the pan using the parchment overhang. Cut into 16 squares by cutting down the middle, then rotating the bars 45º and cutting down the middle again. Wipe the knife clean in between cuts to get clean cuts. Now cut each of the halves in half again, rotating the cookie bars 45º and repeating. You will end up with 16 evenly sized squares.