Preheat the oven to 180° C .
Beat the egg yolks with the sugar in a bowl until pale and stir in the yogurt and cornstarch gradually.
In a separate bowl beat the egg whites until they form stiff peaks and fold them gently into the mixture.
Grease a soufflé dish (21 cm / 8 inch) with butter and dust with sugar to prevent sticking. Fill it up to 3/4 of its height.
Set the dish in a slightly larger pan half-filled with water and bake for about 25 minutes until the soufflé puffs right up and turns golden.