Preparation Steps

• Line an 8-inch square pan with a parchment sling, leaving overhang on two sides; lightly butter the parchment.
• Chop the peppermint patties into 1/2-inch pieces and spread them on a plate. Freeze for 10 minutes to keep the mint layer from smearing during mixing.
• Make the white swirl first: Set a heatproof bowl over a small saucepan of barely simmering water (bowl should not touch water). Add the white chocolate chips, 1/4 cup sweetened condensed milk, and 1 tbsp butter. Stir over low heat until smooth, 2–3 minutes. Turn off the heat and leave the bowl over the warm pot to keep fluid while you make the dark base.
• For the dark chocolate fudge, place dark chocolate chips, 3/4 cup sweetened condensed milk, and 2 tbsp butter in a medium saucepan over low heat. Stir continuously until completely melted and glossy, about 3–4 minutes. Remove from heat and immediately stir in marshmallow creme and vanilla until evenly combined.
• Fold half of the frozen peppermint patty pieces into the warm dark fudge. Scrape about two-thirds of the dark fudge into the prepared pan and smooth into an even layer.
• Scatter the remaining peppermint patties evenly over the surface, pressing lightly so they adhere. Dollop spoonfuls of the warm white chocolate mixture over the top, then spoon the remaining dark fudge in the gaps. Swirl the two chocolates together with a skewer or butter knife using 3–4 gentle figure-eight passes. Tap the pan firmly on the counter to level, then sprinkle with flaky sea salt.
• Set at cool room temperature until firm, 2–3 hours, or refrigerate 60–90 minutes for a quicker set. Lift out using the parchment sling and cut into 36 bars with a warm, dry knife, wiping between cuts.