In a medium pot, heat over low the milk, cream and lemon rind, until very hot but do not boil. Remove from heat and let cool to warm.
In a medium pot add yolks and sugar, whisk together until combined, then add flour and vanilla and whisk again until smooth, place the pot over medium / low heat, pour the warm milk/cream through a sieve to remove the lemon rind (discard), whisk continuously until thickened. Move to a glass bowl, cover with plastic wrap (make sure wrap touches the cream mixture) and let cool, approximately 1-2 hours or refrigerate for about 30 minutes.
Pre-heat oven to 350° (180° celsius) Grease and flour a 9 inch (23-24 cm) cake pan (a springform pan if you have it).
In a medium bowl whisk together flour, salt, baking powder and zest, set aside.
In a large bowl on medium speed beat the eggs, yolk and sugar until pale and creamy, approximately 5 minutes, then beat in the milk, vegetable oil and vanilla until combined, then add the whisked flour mixture and combine.
Pour the batter into the prepared cake pan and spoon 3/4 of the pastry cream on top of the batter, do not stir. Bake for approximately 40-45 minutes. Check the cake at about 30-35 minutes and if browning on top too much, cover with foil and continue baking until done. Test with a toothpick for doneness. Let cool completely before dusting with powder sugar. Enjoy!
To make Cake flour, from 1 cup of all purpose flour remove 2 tablespoons and replace with 2 tablespoon of corn starch, sift to combine.For room temperature eggs and milk, remove from the fridge 30-45 minutes before using.You are going to have some leftover pastry cream, it can be eaten as is or used to fill tart shells.