In the bowl of a stand mixer, fitted with paddle attachment, place butter with sugar, lemon zest and salt.
Knead until you get a creamy mixture.
Add sifted flour with baking powder.
Knead on speed 1 until you get a sandy mixture.
Add egg yolks and knead until you get a smooth mixture.
Stop the a dough is formed.
With your hands, shape the dough into a ball and divide into 2 equal parts.
Between 2 sheets of baking paper, roll out the dough 4 mm thick and refrigerate overnight.
Then, from the rolled dough, with an 8 inches - 20 cm. pastry ring, cut out 2 discs and refrigerate, covered with cling film.
Prepare pastry cream, mixing egg yolks, sugar, vanilla seeds and corn starch.
Heat the milk over medium heat and bring it to a simmer.
Pour over the egg mixture and mix to combine.
Heat the custard base, over medium heat, while whisking vigorously until it starts to thicken and is cooked through.
Pour the custard into a baking sheets, cover the surface with plastic wrap, making sure the plastic wrap is touching the whole surface and refrigerate for about 2 hours.
With a whisk, soften pastry cream, add dark Rum and mix until smooth.
Pour the pastry cream in a pastry bag with a 12 mm nozzle and pipe a custard spiral over a dough disc, leaving 1 inch from the edge.
With water, moisten the edges of the dough disc and place the second dough disc to seal.
With a buttered 7 inches - 18 cm pastry ring, cut out the excess of dough, leaving the cake inside the ring.
Brush the surface with egg yolk and refrigerate 20 minutes.
Heat fan oven at 320F - 160F.
Brush once again the surface with egg yolk and with a fork, create a nice pattern on the surface of the cake.
With a skewer, make here and there some holes and bake for 40/45 minutes.
Let completely cool inside the ring before removing from the mould.