Preheat oven to 350 degrees. Line an 8x8 baking dish with foil or parchment paper and spray with non-stick cooking spray.
In a medium bowl, stir together the melted butter, brown sugar, and vanilla. Stir in the egg until well combined.
Add the flour and salt to the wet ingredients and stir until well combined. Gently stir in 3/4s of the candy. The batter will be thick and sticky.
Spread the batter into the prepared pan. Press the remaining m&ms over the top.
Bake for 20-25 minutes. The center should be barely set.
Cool before cutting.
Video
Notes
You may use any variety of m&ms that you prefer. Adding pastel sprinkles to the batter makes these even more fun and festive!Store in an air-tight container for up to 5 days.