For the pastry cream: Boil the milk with the lemon zest. Take the pot off the heat and strain with a fine mesh strainer.
Mix the superfine sugar, flour and eggs together. Then pour the boiled milk over the mixture. Return to the heat. Let the pastry cream thicken a few minutes while stirring continuously.
For the souffle: Preheat the oven to 430 degrees F/22 degrees C. Grease the souffle dishes. Then pour a little bit of sugar in and swirl, to coat the dishes.
Beat the egg whites with the superfine sugar until stiff peaks form. At the last moment add the potato starch. Add the liqueur to the pastry cream and slowly add in the egg whites. Fill the souffle dishes and place them immediately in the oven for 13 minutes.
After removing from the oven, immediately sprinkle the souffles with the confectioners' sugar and serve straight away.