For the cake: Place an oven rack in the center of the oven. Preheat the oven to 350 degrees F. In a large mixing bowl, combine the egg whites, salt and cream of tartar. Using an electric hand mixer, beat on medium speed until frothy. Add the vanilla extract, lemon juice, and lemon zest. Beat on high speed until soft peaks form, about 2 to 4 minutes. Gradually add the sugar and beat until the mixture forms stiff peaks, about 2 minutes longer. Sprinkle the flour over the egg white mixture. Using a spatula, carefully fold in the flour until incorporated. Spoon the batter into an ungreased 10-inch nonstick angel food cake pan. Bake for 30 to 40 minutes until the top is browned and a skewer inserted halfway between the inner and outer wall of the pan comes out clean. Invert the pan on a wire rack to cool completely, about 1 hour. For the sauce: In a small bowl whisk together the yogurt, honey, and lemon juice until smooth. Pour into a serving pitcher. Using a spatula, loosen the cake around the sides and middle. Release the cake and place on a serving plate. Cut into slices and drizzle with the sauce. Garnish with lemon zest and mint sprigs.