Butter four small soufflé dishes and sprinkle with sugar. Chill in the freezer.
Whisk quark with egg yolks, vanilla and lemon zest. Beat egg whites with sugar until stiff, then stir in cornstarch. Fold egg whites into quark mixture. Divide mixture among soufflé dishes. Bake soufflés in an oven preheated to 200°C (approximately 400°F), about 20 minutes. If desired, invert soufflés onto dessert plates and serve dusted with powdered sugar.