Heat the oven to 180°C. Grease and flour a 24 cm round springform pan and place in the fridge. Sift the flour into a bowl.
Add the sugar and eggs to a large bowl, whip ribbon-like using an electric mixer, until the batter is light and foamy. Sprinkle small quantities of flour and mix gently in with a spatula so as not to create lumps. Pour the batter into the mould and bake for about 22-25 minutes, until the sponge is lightly browned and comes about 2 mm away from the sides of the moulds. Remove the cake from the oven and turn upside down onto a wire rack to cool.
Spray the surface and edges with the liqueur, using a brush. Let stand for 10 minutes before cutting. Meanwhile, start to make the coconut filling. Put the coconut milk into a saucepan and warm it without bringing to a boil. Mix the egg yolks and sugar in a bowl, amalgamating thoroughly with a fork, then add the flour and mix again. Very slowly pour the hot milk into the bowl with the eggs, stirring with a whisk until all the coconut milk has been absorbed.