Heat the white chocolate in the microwave for 30-second bursts, stirring in between, until melted.
Fold white chocolate and raspberries in mascarpone mixture.
Spread mixture onto a slice of bread and then place a second slice on top to make a sandwich. Repeat for remaining slices of bread. Set in the fridge.
Put remaining 3 oz of white chocolate into a bowl. Heat heavy cream until simmering and then pour over the chocolate. Allow to sit for 5 minutes and then stir together. Put white chocolate sauce into the fridge to thicken.
Whisk together milk, eggs, and cinnamon.
Heat griddle. Melt 1 tbsp butter on the giddle if not a non-stick surface.
Quickly dip both sides of each sandwich into milk and egg mixture. Place on hot griddle and cook each side until golden brown, approximately 3-5 minutes.
Plate French toast and drizzle with white chocolate sauce. Top with powdered sugar and maple syrup, if desired.
The ideal bread for French toast is thickly sliced French bread. However, any thick sliced bread will work well. Plain white sandwich bread isn't the best option but will work if dipped very quickly.