Bake the cake, according to package directions, in two 9 inch cake pans. Cool completely
Prepare the frosting: In a large mixing bowl, beat the heavy cream for 3 minutes, until stiff peaks form.
Cover and place in refrigerator.
In a separate large mixing bowl, place the cream cheese, mascarpone cheese, powdered sugar, and vanilla.
Beat until completely blended, about 2 minutes. Make sure to scrape the sides and bottom of the bowl halfway through the beating time.
Fold whipping cream into cream cheese mixture just until all is mixed together.
Cover and place in refrigerator for at least 15 minutes.
Clean all the berries and pat dry.
Add 1 teaspoon water to 1/2 cup strawberry jam. Stir until smooth.
Place one layer of cake on a cake plate with the bottom of the cake facing up.
Spread half of the strawberry jam mixture on cake layer. The layer will be thin.
On top of the jam, spread a thick layer of frosting. Frosting layer should be at least 1/2 inch thick.
Top frosting with sliced berries.
Place remaining cake layer on top of the first one, this time with the top up.
Spread remaining strawberry jam mixture over top of cake.
Follow with a thick layer of frosting.
Using a flat spatula, frost the sides of the cake.
Garnish the top of the cake with more sliced and/or whole berries and fresh mint.
This cake slices and serves beautifully if it is very cold. Store in refrigerator until ready to serve.
Stays fresh in an air-tight cake carrier, in the refrigerator, for up to three days.