1. Line an 8×8-inch baking dish with parchment paper, leaving overhang on the sides for easy lifting.

2. In a microwave-safe bowl, combine 2 cups semi-sweet chocolate chips, ½ can of sweetened condensed milk, and 1 tablespoon of butter. Microwave in 30-second intervals, stirring each time, until melted and smooth. Stir in ½ teaspoon vanilla extract.

3. In another microwave-safe bowl, combine 1 cup white chocolate chips, remaining ½ can sweetened condensed milk, 1 tablespoon butter, and 1½ cups mini marshmallows. Microwave in intervals, stirring frequently, until smooth and melted. Stir in ½ teaspoon vanilla extract.

4. Pour half of the chocolate mixture into the pan and spread evenly. Add spoonfuls of the marshmallow mixture. Repeat with remaining chocolate and dollops of marshmallow mixture.

5. Use a toothpick or knife to swirl the layers together in a marbled pattern.

6. Chill in the refrigerator for at least 2 hours until firm. Lift out of the pan and cut into squares.