Prepare the Baking Pan: Line an 8x8-inch baking pan with parchment paper, greasing lightly for easy removal.

Melt the Chocolate Mixture: Combine chocolate chips, sweetened condensed milk, and butter in a saucepan; stir over low heat until smooth.

Add the Vanilla Extract: Remove from heat and stir in vanilla extract until fully combined.

Create the Marshmallow Swirls: Spoon marshmallow creme over the chocolate mixture and gently fold to create swirls.

Pour into the Prepared Pan: Transfer the mixture to the lined pan, leveling the surface and enhancing swirls.

Chill the Fudge: Refrigerate for 2-3 hours until firm and set.

Slice and Serve: Lift out of the pan and slice into squares using a warm knife for clean edges.