Preheat the oven to 350 degrees.
Line a 9X9 inch baking pan with parchment paper, and set aside.
In a large bowl mix together the brown sugar and melted butter until combined.
Mix in the egg and vanilla, set aside.
In a medium sized bowl combine the flour, salt, baking powder, and baking soda.
Slowly add the dry ingredients to the wet, working in small batches, stirring until thoroughly combined after each until it's all mixed in.
Stir in 2/3 cup M&M's.
Spread into your prepared dish, and sprinkle with the remaining M&M's.
Bake for 25 to 30 minutes, or until a toothpick inserted comes out clean.
Allow to cool completely before cutting.