First, make the cake layers. Pre-heat the oven to 350 and line two 9 inch round cake pans with parchment paper. Then spray the pans well with cooking spray. Combine flour, baking powder and salt in a bowl and whisk them together to aerate them. Combine the milk, eggs, egg yolk, lemon zest, coconut extract and distilled white vinegar in another bowl and whisk that all together until smooth. Set up a stand mixer and combine the sugar and butter in its bowl. Beat them together thoroughly until smooth and fluffy. Slowly add in 1/3 of the dry ingredients, followed by half of the wet ingredients. Let each of them get fully incorporated before adding the next 1/3 of the dry ingredients and remaining wet ingredients. Finish with the dry ingredients just until a lovely cake batter forms. Divide the batter evenly among the two prepared cake pans and bake the cake layers for about 30 minutes. The cakes will be slightly sticky when they come out but a toothpick should come out pretty cleanly when inserted in the center. Let the cakes cool for 30 minutes before turning them out onto large plates to finish cooling completely.

While the cake layers bake and cool, make the Swiss meringue buttercream. You will need to wash out the stand mixer bowl and paddle attachment first. Then it's meringue time! Set a small pot filled halfway with water on the stove and bring it to a gentle simmer. Combine the egg whites, sugar and salt in a heat proof bowl and set the bowl over the simmering water. Whisk the mixture until the egg whites warm up and the sugar dissolves. It will be pale white, thick and smooth and should take just a minute or two. Transfer the mixture to the stand mixer bowl and use the whisk attachment to beat the egg whites until they are stiff and fluffy. Switch to the paddle attachment and beat in the softened butter 1/2 a stick at a time. It will become a luscious, thick buttercream. Finally, beat in the rum thoroughly. Set the frosting aside until you are ready to assemble the cake.

When the cake layers are completely cool, assemble the cake. Place the first layer on a pretty cake plate with the flat side down, then spread the lemon curd in a thick layer on it. Place the second layer on top with the flat side up. Frost the cake on top and all around the sides with an offset spatula and smooth it out as best you can. There is plenty of buttercream to work with in case of any needed patch jobs. Sprinkle the shredded coconut, chopped hazelnuts and lemon zest on top to decorate it. Then set it in the refrigerator until you are ready to serve it! Enjoy!