Line an 8×8-inch square pan with parchment paper, leaving overhang on two sides, and lightly grease the parchment.
In a medium saucepan over low heat, combine chocolate chips, sweetened condensed milk, and butter. Stir constantly until smooth and glossy, 5–7 minutes.
Remove from heat and stir in vanilla extract until evenly incorporated.
Dollop marshmallow creme across the warm chocolate mixture. Fold gently 2–3 times—do not overmix—to create distinct swirls.
Pour the mixture into the prepared pan. Use a butter knife or skewer to lightly drag through the surface, enhancing the marble effect. Sprinkle flaky sea salt if using.
Refrigerate until firm, 2–3 hours. Lift the slab out using the parchment, then slice into 16 squares with a warm knife (dip in hot water and wipe dry between cuts).