In a large bowl, beat the mascarpone, heavy cream, powdered sugar, and imitation rum with an electric mixer until stiff peaks form.
Melt the white chocolate chips in the microwave, stirring every 30 seconds until smooth. Let cool slightly.
Gently fold the melted white chocolate into the mascarpone mixture. Set aside.
Make the raspberry mixture:
In a food processor, blend thawed raspberries and powdered sugar until smooth and jam-like.
Assemble the tiramisu:
Spread half of the raspberry mixture evenly into a 9x13-inch baking dish.
Add a single layer of 22 ladyfingers.
Spread half of the mascarpone cream over the ladyfingers.
Repeat with remaining raspberry mixture, remaining ladyfingers, and top with the rest of the mascarpone cream.
Sprinkle grated white chocolate on top.
Chill and serve:
Cover tightly with plastic wrap and refrigerate for at least 6 hours or overnight.
Slice and serve chilled.
Notes
🥡 STORAGE: Store Covered: Keep tightly covered or in an airtight container.Refrigerate: Store in the refrigerator for up to 4 days.♨️ REHEATING: Not Recommended: This dessert is best served chilled and should not be reheated.❄️ FREEZING: Freeze Assembled: Wrap tightly in plastic wrap and aluminum foil.Storage Time: Freeze for up to 2 months.Thaw Properly: Thaw overnight in the refrigerator before serving.💡 TIPS FOR BEST RESULTS: Use Cold Ingredients: Cold mascarpone and cream whip better.Cool the Chocolate: Let the melted white chocolate cool to room temperature before folding it in.Chill Overnight: This gives the tiramisu time to set properly.Layer Smoothly: Spread each layer evenly for the best presentation.🍳 ALTERNATE COOKING METHODS: No alternate cooking methods are needed for this no-bake dessert.♻️ LEFTOVERS: Serve Cold: Enjoy leftovers straight from the fridge.Better Over Time: Flavors deepen and improve by the next day.Great for Sharing: Leftovers make a beautiful dessert to serve guests.