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.)
Make Buttercream:
1. 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.
2. 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.
To Assemble Cake:
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