Pour heavy cream into a large glass measuring cup. Microwave on high power until boiling, 1 1/2 to 2 minutes (stop earlier if necessary to prevent cream from spilling over). Add white-chocolate chips, let stand for 1 minute and then stir with a fork until completely smooth. Stir in crème de cacao, if using, lemon juice and salt until thoroughly combined.
Warm vanilla ice cream in its container in microwave for 15 to 20 seconds, until just easy to scoop; repeat with raspberry sorbet. Using an ice cream scoop, fill scoop halfway with vanilla ice cream and then scoop up raspberry sorbet to fill ice cream scoop. Fill 6 sundae dishes with two of these scoops each.
Drizzle white-chocolate sauce over sundaes and garnish with raspberries.