Preheat oven to 350 degrees.
In a large bowl, combine the melted butter and brown sugar and mix with a wooden spoon for 1 minute until combined. Add the vanilla and eggs, making sure to scrape down the sides of the bowl, and mix until fully incorporated.
Add the dry ingredients into the bowl and slowly mix until just combined. The batter will be a bit stiff, but that's normal. Fold in 1 cup of the Milk Chocolate M&M’s.
Spray a 9x9 metal baking pan (see note above) with baking spray and line it with parchment paper. Spoon the batter into the prepared baking sheet and sprinkle with the remaining Milk Chocolate M&M’s and Maldon sea salt if desired and transfer it into the oven. Bake for 35-40 minutes. Once the bars have baked, remove them from the oven and let cool for at least 2 hours before cutting and serving.