Step 1Heat the oven to 350 degrees. Spread the walnuts on a baking sheet and toast them until fragrant, 5 to 6 minutes. Do not let them brown. Cool. (Can be done ahead and frozen. Return to room temperature before using.)
Step 2Place the chocolate, butter, sugar and milk in a microwave-safe dish that holds 3 cups. Cook on 10% power for 1 minute stirring once after 30 seconds. Stir until smooth. Stir in the baking powder and vanilla. (This can be made a week ahead and refrigerated or frozen up to 1 month. Let it come to room temperature before reheating.)
Step 3Place 2 scoops of gelato into each of 2 shallow dessert plates. Spoon the hot sauce over, dividing evenly. Sprinkle 2 tablespoons each of white chocolate and walnuts over both sundaes. Serve immediately.