Preheat your oven to 350 degrees Fahrenheit.
In a large mixing bowl, use an electric hand mixer or a standing kitchen mixer to combine the butter, brown sugar, and granulated sugar.
Add the egg and vanilla and mix to combine.
Add the all purpose flour and baking soda and mix to combine again.
Then add the M&Ms into the mixture and mix by hand until evenly distributed.
Press the dough into the bottom of an ungreased 9x13 glass pan. Bake for 25-30 minutes until the top has just begun to brown and is no longer glossy. The bars will continue to bake in the pan.
Allow the bars to rest in the pan at room temperature for 30 minutes, then cut into bars, and continue to cool the rest of the way on a cooling rack. Serve and enjoy!