Finely chop the walnuts into 1/4-inch pieces, then set aside 1/2 cup of the chopped walnuts in a shallow bowl for coating—this will be your truffle exterior. Roughly chop the remaining walnuts and the raisins, keeping them separate for now. Having everything prepped and ready makes the assembly process smooth and prevents the chocolate from setting while you're still chopping.

Combine the finely chopped dark chocolate and unsalted butter in a heatproof bowl. Set the bowl over simmering water (double boiler method) or microwave in 20-second intervals, stirring between each pulse, until completely melted and smooth. I use Kerrygold butter here because its rich, creamy quality adds incredible depth to these truffles. Once melted, remove from heat and let cool for 1-2 minutes before proceeding.

Stir the room-temperature dulce de leche into the melted chocolate mixture until fully incorporated and smooth. The dulce de leche should be at room temperature for easier mixing—if it's too cold, it will seize the chocolate. Fold in the roughly chopped walnuts and raisins until evenly distributed throughout the mixture.

Transfer the mixture to the refrigerator and chill for 15-30 minutes, until it firms up enough to hold its shape when rolled but is still pliable. The mixture should be cool to the touch and have a fudgy consistency. Check at the 15-minute mark—you want it firm enough to roll but not so hard it becomes difficult to work with.

Remove the chilled mixture from the refrigerator and, working quickly, use a small spoon or your hands to scoop and roll the mixture into 1-inch balls. I find that slightly damp hands prevent sticking without adding moisture. Immediately roll each ball in the reserved 1/2 cup of finely chopped walnuts, pressing gently so the coating adheres well. Work efficiently—the truffles will soften as they sit at room temperature.

Transfer the coated truffles to a parchment-lined plate and return to the refrigerator for at least 1 hour to firm up completely. This final chill ensures the truffles hold their shape and develop a nice firm exterior. Once set, store in an airtight container in the refrigerator for up to 2 weeks, or freeze for longer storage.