FOR THE CAKE:
2 1/2 cups cake flour
1 Tablespoon baking powder
1/2 teaspoon salt
1 1/4 cups buttermilk
4 large egg whites
1 1/2 cups granulated white sugar
2 teaspoons freshly grated lemon zest
1/2 cup (1 stick) unsalted butter, at room temperature
1/2 teaspoon pure lemon extract
FOR THE BUTTERCREAM:
1 cup granulated white sugar
4 large egg whites
1 1/2 cups (3 sticks) unsalted butter, at room temperature
1/4 cup freshly squeezed lemon juice (from 2 large lemons)
1 teaspoon vanilla extract
2/3 cup seedless raspberry preserves, stirred vigorously until spreadable
1. Center a rack in the oven and preheat oven to 350°F. Butter two 8 or 9-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.
2. To Make Cake: Sift together flour, baking powder and salt. Whisk together milk and egg whites in a medium bowl. Put sugar and lemon zest in a mixer bowl or another large bowl and rub them together with your fingers until the sugar is moist and fragrant. Add the butter and, working with your fingers until the sugar is moist and fragrant. Add butter and, working with the paddle or whisk attachment, or with a hand mixer, beat at medium speed for a full 3 minutes, until the butter and sugar are very light. Beat in the extract, then add one third of the flour mixture, still beating on medium speed. Beat in half of the milk-egg 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 homogenous, 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.
3. Bake for 30 to 35 minutes, or until the cakes are well risen and springy to the touch- a toothpick inserted into the centers should come out clean. Transfer cakes to cooling racks and cool for about 5 minutes, then run a knife around the sides of the cakes, unmold them and peel off the paper liners. 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 2 months.)
4. To Make Buttercream: Put sugar and egg whites in a mixer bowl or other large heatproof bowl; fit the bowl over a pan of simmering water and whisk constantly, keeping the mixture over the 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 heat.
5. Working with the whisk attachment or with a hand mixer, beat the meringue on medium speed until it is cool, about 5 minutes. Switch to the paddle attachment if you have one, and add the butter one stick at a time, beating until smooth. Once all the butter is in, beat the buttercream on medium -high speed until it is thick and very smooth, 6 to 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 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.
6. To Assemble Cake: Using a sharp serrated knife and a gentle sawing motion, slice each layer horizontally in half. Put one layer cut-side-up on a cake plate surrounded by strips of waxed paper. Spread it with a light layer of buttercream. Spread the buttercream with 1/3 of the preserves. Top with another layer, spread with buttercream and preserves and then do the same with the third layer (you’ll have used all of the jam and have buttercream left over.) Place the last layer cut-side-down on top of the cake and use the remaining buttercream to frost the sides and the top. Decorate top with raspberries.