1. Line an 8×8-inch square pan with parchment paper, leaving some overhang on the sides for easy removal. Lightly grease the paper.

2. In a medium saucepan over low heat, combine chocolate chips and sweetened condensed milk. Stir continuously until melted and smooth.

3. Remove from heat and stir in the vanilla extract.

4. Pour about two-thirds of the chocolate mixture into the prepared pan and spread evenly.

5. Drop spoonfuls of marshmallow crème over the surface, then add the remaining chocolate mixture in between.

6. Use a butter knife or offset spatula to gently swirl the two mixtures into a marbled pattern. Do not overmix.

7. Chill in the refrigerator for at least 2 hours, or until fully set.

8. Once firm, lift the fudge out of the pan using the parchment and cut into squares with a warm knife.