Roast plums: Preheat oven to 425 degrees F.
In a large bowl combine plums with brown sugar and pepper. Transfer to a rimmed baking sheet. Roast until plums are soft and juice is bubbling, about 16 to 18 minutes. Remove from oven. Transfer plums and their juices to large bowl. Cool and chill in refrigerator.
Make citrus curd: While plums roast, in a heavy large saucepan whisk the juices, 1 cup sugar, zests, and eggs in a heavy saucepan. Add butter and cook over moderately low heat, whisking frequently, until thickened. Force through a fine-mesh sieve into a wide shallow dish; cover surface with plastic wrap. Cool completely, stirring occasionally, about 30 minutes. Chill until completely cold (about 30 minutes).
Make basil syrup: Combine the water, 1 cup sugar, and basil in a medium saucepan. Bring to boil, stirring until sugar dissolves. Cover and steep 15 minutes. Strain through a fine mesh sieve into a glass measuring cup. Cool completely in freezer until cold.
For filling: In a large bowl, beat the mascarpone, 1/4 cup sugar, 1/4 cup whipping cream, and vanilla until smooth with electric hand mixer. In a stand mixer (with whisk attachment), beat the remaining 2 cups whipping cream until peaks form. Fold whipped cream into mascarpone mixture.
Puree 1 cup of the roasted plums and 1/4 cup basil syrup in a blender or food processor. Transfer to medium bowl and stir in remaining roasted plums.
Assembly: Arrange 1/3 of cake cubes in bottom of 3 1/2 quart glass trifle dish. Drizzle with 1/3 of remaining basil syrup. Spoon 1/3 of mascarpone filling over cake in dollops; spread to sides of dish. Spoon half of plum mixture over; spread to sides of dish. Spoon half of citrus curd over plum mixture in dollops, then spread to sides of dish. Repeat layering with 1/3 of cake cubes, 1/3 basil syrup, 1/3 of mascarpone filling, and remaining plum mixture, then remaining cake cubes, basil syrup, and remaining citrus curd. Spread remaining mascarpone filling over. Cover and chill at least 1 hour. Serve sprinkled with nuts and garnished with basil sprigs.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.