Heat oven to 350 °F (175 °C). Lightly spray an 8x8" baking pan with cooking spray.
Using an electric mixer, in a large bowl beat the egg whites at high speed, with the cream of tartar and salt until stiff peaks for. About 5 minutes.
Add 1/2 teaspoon of the vanilla extract and beat to combine. Add 1/3 cup of the sugar substitute and beat until the mixture forms stiff peaks. Sift 2 tablespoons of the flour over the beaten egg whites and fold gently to incorporate. Repeat twice until all the flour is folded into the egg white mixture.
Pour the batter into your prepared pan and gently smooth with an off-set spatula. Bake turning once half way through the cooking time and until your cake is golden and the tester comes out clean. About 20 minutes.
Cool thoroughly, do not remove from the pan until this is totally cool, it'll fall apart. In a small bowl combine the ricotta, whipped cream topping, remaining 2 teaspoons of sugar substitute, and remaining 1/4 teaspoon of vanilla extract.
Cut the cake in half vertically down the middle to make two 4x8" pieces. Place the halves on a flat work surface. Drizzle 2 tablespoons of the espresso onto each half. Spread half of the ricotta mixture on to one of the halves and dust with half of the cocoa powder.
Top with the remaining cake half; spread the top with the remaining ricotta mixture and dust with remaining cocoa powder. Using a serrated knife, gently cut cake crosswise into 4 slices and serve with the mint leave for garnish if using.
Based on recipe from the South Beach Diet Taste of Summer Cookbook.