Lightly coat an 8x8 baking pan with cooking spray.
In a large bowl, with an electric mixer at high speed, beat egg whites, cream of tartar, and salt until soft peaks form, about 5 minutes. Add 1/2 tsp of the vanilla and beat to combine. Add 1/3 cup of the sugar substitute and beat until stiff peaks form. Sift 2 TB of the flour over beaten egg whites and gently fold to incorporate. Repeat twice with remaining flour until all the flour is incorporated.
Pour batter into the pan and gently smooth the top. Bake, turning once halfway through, until cake is golden and a tester inserted into the center comes out clean, about 20 minutes. Cool completely.
In a small bowl, combine ricotta, whipped topping, remaining 2 tsp sugar substitute, and remaining 1/4 tsp vanilla. Cut cake in half vertically down the middle to make two 4x8 pieces. Place the halves on a flat work surface. Drizzle 2 TB of the espresso onto each half.
Spread half of the ricotta mixture onto one of the cake halves and dust with half of the cocoa powder. Top with remaining cake half; spread the top with remaining ricotta mixture and dust with remaining cocoa powder. Using a serrated knife, gently cut cake crosswise into 4 slices. Eat & Enjoy!