Prepare the pan
Line an 8×8-inch (20×20 cm) pan with parchment paper, leaving overhang.
Melt the fudge base
In a saucepan over low heat, combine white chocolate, condensed milk, butter, and salt. Stir constantly until fully melted and smooth.
Flavor the base
Remove from heat and stir in vanilla extract.
Add raspberry swirl
Pour most of the fudge mixture into the pan. Spoon raspberry puree or preserves on top.
Swirl
Use a knife or skewer to gently swirl the raspberry through the fudge.
Chill
Refrigerate for 3–4 hours, or until completely set.
Slice & serve
Lift out using parchment and cut into small squares.