Grease 4 soufflé molds with butter and sprinkle with sugar. Rinse the lemon in hot water, wipe dry and finely grate the zest. Squeeze juice from the lemon.
Separate the eggs. Beat the egg whites with the salt until stiff.
Sprinkle 2-3 tablespoons sugar into the egg whites and beat again until stiff. Beat the egg yolks with the remaining sugar, vanilla sugar and lemon juice, and stir in the lemon zest. Add the quark and stir well until smooth.
Fold the egg whites into the egg yolk-quark mixture and pour into the soufflé molds. Place the molds in a baking dish and pour as much boiling water into the dish such that the molds sink about halfway. Bake the soufflés in a preheated oven at 220°C (fan: 200°C, gas mark 4) (approximately 420°F) for about 20 minutes.
Remove from the oven, duste the baked soufflés with powdered sugar, and serve immediately.