Place the eggs and sugar in a bowl and mix until you obtain a smooth and homogeneous cream. Mix in the flours gradually and, if the dough is too dry, add 3 tablespoons of cold milk. In a separate pan, boil the remaining milk with the lemon or orange zest and then turn off the heat. Add the milk to the mixture and mix gently, taking care not to form lumps. Place the pan with the cream on the stove (low heat) and bring it to the boil. Once the cream has thickened, move it into a glass container and let it cool before using it.