In a heat-proof bowl set over simmering water, heat the cream until hot to the touch but not boiling and turn off the heat. Add the butter and 12 ounces of the chocolate to the hot cream. Stir until the chocolate melts. Meanwhile, mince the raisins and combine them with the rum in a small bowl. Stir the raisins and rum mixture into the melted chocolate mixture, cover with plastic wrap and set aside at room temperature for 6 hours or overnight.
The next day, using a very small (1 1/4-inch) ice cream scoop (you can also use 2 spoons), scoop balls of the mixture onto sheet pans lined with parchment paper. Refrigerate for at least 30 minutes.
When the chocolate is cold, place 10 ounces of the remaining chocolate in a small glass bowl and heat it in a microwave on high for 20 seconds exactly. Stir the chocolate and continue to heat it in the microwave for 20 seconds at a time, stirring in between, until the chocolate is smooth and shiny. This will take 5 or 6 times, depending on your microwave. Add the remaining 4 ounces of chocolate and stir until smooth. The chocolate should be 85 degrees F to 90 degrees F on an instant-read thermometer. If it¿s too cool, heat it briefly in the microwave. You¿ll want it to be pourable.
Using 2 spoons, pick up a chilled ball in one spoon and warm chocolate in the other and roll them around together until the chilled ball is completely coated in warm chocolate (don¿t drop the ball into the chocolate or the warm chocolate will cool off!). Place the coated truffle back on the parchment paper. Continue coating the balls until they¿re all covered with milk chocolate. Allow to sit at room temperature until the chocolate hardens. Serve at room temperature.