Preheat oven to 350 degrees
Melt butter in a small saucepan on medium heat. Allow butter to come to a mild boil, just slightly bubbling. The butter will foam and start to crackle. After about 5 to 6 minutes the butter will start to settle and you'll smell a strong, nutty aroma. Watch closely because the butter will turn from yellow to an amber shade. Take the saucepan off the heat and pour the butter through a fine mesh strainer into a small container. Place butter in the refrigerator for 5 minutes to cool.
In a separate bowl combine light and dark brown sugars and cooled brown butter. Using a mixer, cream the butter and sugars together. Mixture will be dark,moist and clumpy. Add in egg and vanilla extract and continue mixing, texture will become smooth.
Slowly add in flour, baking powder and salt. Mix everything together until thoroughly combined. Fold in chocolate chunks and M&Ms.
Line a 8x8 inch baking dish with parchment paper. Allow paper to come up the sides of the baking dish for easy transfer once blondies are cooled. Scoop batter into the baking dish and smooth it out with the back of a spoon.Bake blondies for 20-25 minutes. Blondies will seem under done when they first come out but will firm up once they've cooled.
Once blondies are out of the oven allow to cool for 30 minutes before transferring them from the baking dish to a cooling rack. If you transfer them too soon they will crack. Cut into small squares and serve or store in an airtight container.