Step 1: In a large mixing bowl, cream together 1/2 cup (1 stick) softened unsalted butter, 1/2 cup light brown sugar (packed), 1/4 cup granulated sugar, and 1 teaspoon vanilla extract using an electric mixer until light and fluffy, about 2-3 minutes. Scrape down the sides.
Step 2: Add 1 1/2 cups all-purpose flour and 1/4 teaspoon salt to the butter mixture. Mix on low speed until just combined. Avoid overmixing to keep the dough tender.
Step 3: Stir in 1/2 cup mini chocolate chips until evenly distributed. Set this delicious edible cookie dough aside.
Step 4: In a large, heavy-bottomed saucepan, combine 1/2 cup (1 stick) unsalted butter, 1 1/2 cups granulated sugar, and one 14-ounce can sweetened condensed milk. Place over medium-low heat.
Step 5: Cook, stirring constantly, until the butter is melted and sugar is fully dissolved. Bring the mixture to a gentle simmer. Continue simmering and stirring for 5-7 minutes. It will thicken slightly. Remove from heat immediately.
Step 6: Quickly stir in 1 teaspoon vanilla extract, a pinch of salt, and 1 cup of M&M's (reserving 1/2 cup for topping). Mix until well combined. This forms the vibrant base of your mm cookie dough fudge.
Step 7: Pour half of the prepared fudge mixture into your parchment-lined pan. Spread into an even layer with an offset spatula.
Step 8: Carefully dollop spoonfuls of the edible cookie dough over the fudge layer. Gently spread it out to create an even layer.
Step 9: Pour the remaining fudge mixture over the cookie dough layer, gently spreading to cover completely.
Step 10: Immediately sprinkle the reserved 1/2 cup of M&M's over the top, lightly pressing them into the warm fudge so they adhere.
Step 11: Refrigerate the mm cookie dough fudge for at least 2-3 hours, or until completely firm. This is crucial for easy cutting.
Step 12: Use the parchment overhangs to lift the fudge from the pan. Place on a cutting board. Using a sharp, warm knife, cut into 1-inch squares. Store in an airtight container at room temperature for up to 5 days, or in the refrigerator for up to 2 weeks. For longer storage, freeze for up to 2 months.