Preheat the oven to 325F and grease three 8-inch metal cake pans well. Line the bottoms with parchment paper and grease the paper.
In a large bowl, whisk together the coconut flour, sweetener, whey protein, baking powder, and salt. Stir in the eggs, egg whites, melted butter, hemp milk, and extracts until well combined and the batter is smooth.
Divide the batter evenly among the three prepared pans and spread to the edges. Smooth the tops. Bake 25 to 30 minutes, until lightly browned around the edges and the tops are firm to the touch.
Remove from the oven and let cool completely in the pans, then run a sharp knife around the insides of the pans and flip the cake layers out.
While the cakes are baking, combine the berries and water together in a small saucepan over medium heat. Bring to a boil, then reduce the heat to a simmer.
Cook until the berries are soft enough to be easily mashed up. Remove from heat and stir in the sweetener, mashing as you go. Stir in the glucomannan and let sit to thicken.
In a large bowl, beat the cream cheese and mascarpone together until smooth. Beat in the powdered sweetener until well combined.
Beat in the extracts, then beat in 1/4 cup of the heavy cream to lighten the mixture.
In another large bowl, beat the remaining cream until it holds stiff peaks. Fold into the cream cheese mixture until well combined.
To assemble the cake
Place one layer of cake on a cake platter, and spread with 1/2 of the berry syrup. Then spread with 1/4 of the frosting and arrange 1/3 of the berries. Repeat with a second layer of cake.
Place the final layer of cake on top and spread the top and the sides with the remaining frosting (you will not do any berry syrup on this layer). Top with the remaining berries.
Refrigerate at least 1 hour to firm up the frosting.