1. Preheat the oven to 350F (180C).
2. For the cake in a medium bowl, sift together the flour, half of the sugar, and salt and set aside. Place the egg whites in the bowl of a stand mixer fitted with the whisk attachment and whisk on medium speed until foamy. Add 1 tablespoon water, the cream of tartar, and vanilla and whisk until just beginning to form soft peaks.
3. Add the remaining sugar, 1 tablespoon at a time, and whisk until incorporated. Continue whisking on medium speed until almost-stiff peaks form that are glossy and moist, and flop just a bit at the top when the whisk is raised. Sift the flour mixture over the whites, a little at a time, gently folding in the flour with each addition using a rubber spatula.
4. Transfer the batter to an ungreased 10-to-12 cup (2.4-to-2.8-L) tube pan. Smooth the top with a small offset spatula or butter knife, and run the spatula through the batter to break any air bubbles. Bake the cake for 35 to 40 minutes, rotating at the halfway point. The cake is ready when the top is browned and a cake tester comes out with a moist crumb or two. Place the pan upside-down on the neck of a wine bottle to cool completely. Using a small paring knife, release the cake from the sides of the pan and from the middle. Invert the cake right-side up onto a serving plate.
1. For the whipped cream combine the blueberries, granulated sugar, and ½ cup (120ml) water in a small saucepan and cook over medium to medium-high heat until the berries are loose and jammy, about 20 minutes. Let cool completely. If the mixture looks too thick once it has cooled, add 1 or 2 tablespoons of boiling water.
2. Place the cream in the bowl of a stand mixer fitted with the whisk attachment and whisk on medium to medium-high speed until the cream begins to thicket, 2 to 3 minutes. Add the brown sugar and vanilla and continue whisking until medium peaks form. Add the blueberry compote and whisk to combine.
3. Using a large serrated knife and a sawing motion, gently slice the cooled cake in half horizontally. Spread the whipped cream on the cut side of one cake layer, add the top layer, and spread the whipped cream all over the entire cake. Refrigerate for about 1 hour to set up.
4. Serve with a few fresh berries on each plate. The cake will keep refrigerated, lightly covered in plastic wrap, for up to 3 days.