Mix mashed potatoes, egg yolks, sugar, juice of half a lemon and salt in a ladle with a thick bottom. Stir until smooth. Heat over medium heat with constant stirring until boiling. The mass will begin to thicken. The first big burst bubble is a sign that it’s time to take it off the fire. It won’t get any thicker, but it can easily get burned. We got custard, or cottage cheese, as it is called in England. Put the warm custard base in a glass. Gradually adding small pieces of butter, whisk with a blender until smooth. The longer you punch, the tastier – the emulsification process takes place, as in the preparation of homemade mayonnaise. If the cooled cream has a strong oily taste, it simply means that it is poorly mixed. Cover the cream with cling film and refrigerate for 30 minutes.