Step 1: Heat-treat the flour by spreading it on a baking sheet and baking at 350°F (175°C) for 5-7 minutes until it reaches 160°F. Let it cool completely and sift to remove lumps.
Step 2: Line an 8x8 inch square baking pan with parchment paper, leaving an overhang for easy removal.
Step 3: In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy (about 3 minutes).
Step 4: Mix in the vanilla extract and salt until well combined.
Step 5: Gradually add the heat-treated flour and the sweetened condensed milk, alternating between the two and mixing on low speed until a smooth dough forms.
Step 6: Gently fold in 1.5 cups of mini M&Ms using a spatula.
Step 7: Press the mixture firmly into the prepared pan and smooth the top. Sprinkle additional M&Ms on top and press them down slightly.
Step 8: Refrigerate for at least 4 hours, or until firm, before lifting out of the pan and slicing into 1-inch squares.