Get prepped: Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
Melt chocolate: In a medium saucepan over low heat, combine 3 cups semi-sweet chocolate chips, 14 ounces sweetened condensed milk, and 2 tablespoons butter. Stir continuously until melted and smooth.
Add flavoring: Remove from heat and stir in 1 teaspoon vanilla extract.
Fold in mix-ins: Let mixture cool for 2-3 minutes so marshmallows don’t melt completely. Gently fold in 3.5 oz mini marshmallows and 3.5 oz pecans.
Chill: Spread mixture evenly into prepared pan. Top with extra marshmallows and pecans, press gently in. Refrigerate for at least 2 hours, or until firm.
Serve: Cut into squares and enjoy.
