Place rack in the middle of the oven and preheat 350° F. Butter two 9 x 2 inch round cake pans and line the bottom of each pan with a round of buttered parchment or wax paper. Put the pans on a baking sheet.

CAKE: In the medium bowl, sift together the flour, baking powder and salt. Whisk together the sugar and lemon zest in the mixer bowl and rub them together with your fingers until the sugar is moist and fragrant. In a small bowl, whisk together the egg whites and buttermilk.

Add the butter. Using the paddle attachment, beat at medium speed for 3 minutes, until the butter and sugar are very light. Beat in the lemon extract, then add one third of the flour mixture, still beating on medium speed. Beat in half of the buttermilk and egg white mixture, then beat in half of the remaining dry ingredients until incorporated. Add the rest of the milk and eggs beating until the batter is homogeneous, then add the last of the dry ingredients. Finally, give the batter a good 2 minute beating to ensure that it is thoroughly mixed and well aerated.

Divide the batter between the two pans and smooth the tops with a rubber spatula. Bake for 30-35 minutes, or until the cakes are well risen and springy to the touch. A toothpick inserted in the middle should come out clean. Transfer the cakes to cooling racks and cool in the pans for about 5 minutes, then run a knife around the sides of the cakes, unmold them and peel off the parchment. Invert and cool to room temperature, right side up (the cooled cake layers can be wrapped airtight and stored at room temperature overnight or frozen for up to two months).

BUTTERCREAM: Put the sugar and egg whites in the medium heatproof bowl and fit the bowl over a plan of simmering water. Whisk the the mixture constantly over medium heat, until it feels hot to the touch, about 3 minutes. The sugar should be dissolved, and the mixture will look like shiny marshmallow cream. Remove the bowl from the heat.

Working with the whisk attachment, beat the meringue on medium speed until it is cool, about 5 minutes. Switch to the paddle attachment and add the butter a stick at a time, beating until smooth. Once all the butter is in, beat in the buttercream on medium-high speed until it is thick and very smooth, 6-10 minutes. During this time the buttercream may curdle or separate – just keep beating and it will come together again.

On medium speed, gradually beat in the lemon juice, waiting until each addition is absorbed before adding more, and then the vanilla. You should have a shiny smooth, velvety, pristine white buttercream. Press a piece of plastic against the surface of the buttercream and set aside briefly.

ASSEMBLE: Using a sharp serrated knife and a gentle sawing motion, slice each layer horizontally in half. Put one layer cut side up on the prepared cake round. Place it on the cake plate with pieces of wax paper between the cardboard and the plate.

Spread one third of the preserves on the cake. Cover the jam evenly with a thin layer of the buttercream.

Add another layer, spread with preserves and buttercream and then do the same with a third layer. You will have used all the jam and have buttercream leftover.

Place the last layer cut side down on top of the cake and frost the sides and top. Put the remaining buttercream into a piping bag fitted with a rosette decorating tip. Pipe rosettes along the top edge and around the bottom edge of the cake.

Sprinkle the coconut on top of the cake and add the lemon zest strips. Carefully remove the wax paper and serve.