For the potato soufflé, mix cold milk with starch until smooth, heat on the stove and stir for 1 minute. Add butter and let it cool down. Fold in the yolks one by one, add the curd cheese and the grated lemon and orange. Beat the three egg whites with sugar until stiff and mix into the soufflé mixture together with the flour. Butter the soufflé molds and sprinkle with granulated sugar, fill in the mixture.
The soufflé molds should be filled no more than two-thirds full, as the mixture will rise a lot. Steam for 20 minutes. Serve immediately.