Line an 8×8-inch pan with parchment, leaving an overhang.
Caramel swirl: Warm the cream in a small saucepan over low heat. Add caramels, white chocolate chips, and butter; stir 5–6 minutes until smooth and thick. Remove from heat.
Chocolate base: Warm the condensed milk in a second saucepan over low heat. Add chocolate chips, butter, and salt; stir until smooth. Stir in vanilla.
Pour the chocolate base into the pan and level it. Drop spoonfuls of caramel over the top.
Drag a hot, dry knife through the surface in a few S-curves to marble. Don't overmix.
Refrigerate at least 2 hours, until firm.
Lift out by the parchment and slice with a warm knife into 36 squares.
Equipment
8×8-inch square pan — the standard size for thick, sliceable fudge (a 9×9 works too for slightly thinner pieces)
2 small-to-medium saucepans — one for the chocolate base, one for the caramel swirl, running simultaneously (for the microwave method, use 2 microwave-safe bowls instead)