Preheat the convection microwave oven at 180°C.
Sift together the cake flour, salt, and half of the sugar. Set the remaining sugar aside.
In a bowl, use a whisk to combine egg whites, water, vanilla extract, and cream of tartar. After 2 minutes, use a hand mixer. Mix the remaining sugar into the egg mixture, beating at medium speed. Once you achieve medium peaks, add some flour mixture to the foam. Fold in gently using a spatula. Continue folding in the flour until well incorporated.
Carefully spoon the mixture into an ungreased tube pan. Bake the cake for 35 minutes before checking for doneness using a wooden skewer.
Cool upside down on a cooling rack for at least an hour before removing from the pan. Assemble the cake with your desired toppings, such as whipped cream and berries.