2 large eggs plus 3 large egg yolks (save the whites for later!!)
1/2 teaspoon baking powder
1 tablespoon lemon zest
2 cups sugar
1 cup buttermilk
1/4 cup plus 2 tablespoons lemon juice
1 lemon, thinly sliced and seeded (this means very thin! Thinner than I did!)
1. Heat oven to 350°F. Butter and flour two 8-by-2-inch cake pans, tapping out excess flour. In a medium bowl, whisk together flour, baking powder, baking soda, salt, and lemon zest.
2. In a large bowl, using an electric mixer (trusty Kitchenaid!) beat butter and 1 1/2 cups of sugar until light and fluffy. With mixer on low, beat in eggs and yolks, one at a time. Beat in 2 tablespoons of lemon juice. Alternate adding flour mixture and buttermilk, beginning and ending with flour mixture (I did 3 additions of flour, 2 of buttermilk). Mix just until combined.
3. Divide batter between pans; smooth tops. Bake until cakes pull away from sides of pans, 32-35 minutes. (This was really cool and easy to know when it was done, because it did clearly and visibly shrink around the edge a bit where I could see a gap between cake and pan. Great foolproof plan for novice bakers.) Let cool in pans 10 minutes. Run a knife around edges of pans and invert cakes onto a wire rack.
4. While cakes are baking, bring remaining 1/2 cup of sugar and 1/2 cup of water to a boil in a saucepan. Add lemon slices and simmer for 25 minutes. Using slotted spoon, transfer lemon slices to a waxed-paper-lined plate. Stir remaining 1/4 cup of fresh lemon juice into syrup.
2. Cook over medium heat, stirring constantly, until sugar has dissolved, 3-5 minutes.
3. Transfer to a large bowl. Using an electric mixer (Kitchenaid again, with whisk attachment), beat on medium-high until glossy, stiff peaks form, about 3 minutes.
4. Reduce speed to low, add vanilla (lemon!), and beat until just combined. Use immediately.
The only other advice I would give would be to slice the lemons with a mandolin, if you have one, so that they candy properly and stack more prettily on top of the cake. I did it as thinly as I could with a knife, and it wasn't quite thin enough.