Mix flour, sugar, salt and zest in a bowl, and set aside. Combine yeast, water, oil and eggs in the bowl of an electric mixer, and then add the flour mixture and mix for 10 minutes on medium speed. Add the softened butter slowly while mixing until combined.
Cover your dough and set aside for one hour to prove.
Cut and roll the dough into 10 even balls (we weigh them at 40gm each). Set aside on a tray at room temperature to prove until they have increased in size by half.
Deep fry the doughnuts at 180C until golden, and set aside to cool. Dust them in cinnamon sugar and cut a slit in the side ready for filling.
Combine milk, butter and vanilla in a saucepan and bring to the boil. Set aside.
Mix the eggs and sugar until just incorporated. Add the plain flour and whisk, and then the corn flour and whisk.
Pour half of the hot milk mixture over the egg yolks, mix well and return to the pot with the rest of the milk mixture. Cook out over a low heat for 5 minutes, stirring constantly to avoid scrambling the yolks.
Remove the vanilla pod, cover with greaseproof paper and cool.
For Chocolate Cream add 150gm melted dark chocolate at the end.
For Salted Caramel add one cooked 395gm tin of condensed milk per 40gm of base custard (cooked in the tin, in boiling water for three hours). Add flaky sea salt to taste and mix to combine.