1) Place an oven rack in the centre of the oven. Preheat the oven to 180C/Gas 4.
2) In a large mixing bowl, combine the egg whites, salt and cream of tartar. Using an electric whisk, beat on medium speed until frothy.
3) Add the vanilla essence, lemon juice, and lemon zest. Beat on high speed until soft peaks form, about 2 to 4 mins. Gradually add the sugar and beat until the mixture forms stiff peaks, about 2 mins longer.
4) Sprinkle the flour over the egg white mixture. Using a spatula, carefully fold in the flour until incorporated. Spoon the batter into an ungreased 25cm non-stick bundt tin. Bake for 30 to 40 mins until the top is browned and a skewer inserted halfway between the inner and outer wall of the tin comes out clean.
5) Invert the pan on a wire rack to cool completely, about 1 hr.
For the sauce:
1) In a small bowl whisk together the yoghurt, honey, and lemon juice until smooth. Pour into a jug.
2) 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.