Bake cake per box directions in two 8-inch pans. Cool completely.
Melt white chocolate in a bowl over hot water. Let cool slightly.
Sprinkle gelatin over cold water to bloom 5 minutes, then warm until liquid. Stir into melted chocolate.
Whip 1 1/2 cups cream to soft peaks. Fold in mascarpone and powdered sugar, then fold in white chocolate mixture.
Mash raspberries with 2 tbsp sugar and lemon. Fold gently into remaining 1/2 cup whipped cream for a ripple, or spread a layer between cake layers.
Stack cakes with mousse between and on top. Chill 4 hours before serving. Garnish with fresh raspberries.