• Preheat the oven to 350°F. Line an 8-inch springform pan with parchment paper. If your pan is less than 3 inches tall, make a collar with the parchment paper to extend the height of the cake pan. 
• Separate the eggs yolks from the whites and place them in separate bowls. 
• Add the cream of tartar to the egg whites and whip the whites on medium-high speed until they reach stiff peaks. Set the bowl aside.
• Add the sugar to the bowl with the egg yolks and whisk until smooth. Whisk in the vanilla extract and continue beating for about 3 to 4 minutes until pale yellow. 
• Add the yogurt, flour, baking powder, and salt to the egg yolk mixture and whisk until smooth.
• Fold ⅓ of the egg whites into the yogurt batter until well incorporated. Gently fold in the remaining egg whites. 
• Pour the mixture into the prepared springform pan and transfer it to the oven. 
• Bake for 45 to 50 minutes. The cake will puff up very tall and then deflate when it comes out of the oven. 

Raspberry Sauce

• Combine the raspberries, lemon juice and sugar in a saucepan set over medium high heat. Cook and stir until the raspberries break down and get juicy. Bring the mixture to a boil then reduce the heat to a simmer. Cook for 7 to 10 minutes, stirring frequently until slightly thickened. 
• If you want a thicker sauce: In a small dish, combine the cornstarch with 2 tablespoons of water. Add 2 to 3 tablespoons of the raspberry mixture to the cornstarch slurry. Stir well, then stir it into the saucepan. Cook, stirring constantly over medium heat for 1 to 3 minutes or until the mixture is thick. 
• When the cake is cool, serve the raspberry sauce over the yogurt cake.