Step 1: Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides. Set aside.
Step 2: In a large mixing bowl, using an electric mixer, cream together the softened unsalted butter, light brown sugar, granulated sugar, and salt until light and fluffy (about 2-3 minutes).
Step 3: Add the sweetened condensed milk and vanilla extract. Beat on medium speed until fully combined and smooth. Scrape down the sides of the bowl as needed.
Step 4: Gradually add the heat-treated all-purpose flour to the wet mixture. Mix on low speed until just combined and no streaks of flour remain. Be careful not to overmix.
Step 5: Gently fold in the semi-sweet chocolate chips and M&M's candies using a spatula until evenly distributed throughout the cookie dough mixture.
Step 6: Transfer the mm cookie dough fudge mixture to the prepared 8x8-inch pan. Press it down firmly and evenly using the back of a spoon or lightly greased hands. Sprinkle additional M&M's on top, pressing them in gently.
Step 7: Cover the pan loosely with plastic wrap and refrigerate for at least 2-3 hours, or until the mm cookie dough fudge is completely firm. For best results, chill overnight.
Step 8: Once firm, use the parchment paper overhang to lift the mm cookie dough fudge out of the pan. Place it on a cutting board and cut into 1-inch squares. Serve immediately or store in an airtight container in the refrigerator for up to 1 week.