Preheat the oven to 325°F convection or 350°F conventional. Spray cake pans with nonstick spray and line bottoms with parchment.

In a small bowl, rub the lemon zest into the granulated sugar until fragrant and slightly damp.

Sift together cake flour, kosher salt, and baking powder; set aside.

In a stand mixer fitted with the paddle attachment, cream the butter and lemon sugar on medium-low speed for about 3–4 minutes, until lightened and the sugar granules look slightly separated.

Add eggs one at a time, mixing on medium speed after each addition until fully incorporated. Scrape the bowl well after every other egg.

Reduce mixer speed to low. Add the dry ingredients in three additions, alternating with the buttermilk mixed with lemon juice in two additions, beginning and ending with the dry ingredients. Mix just until combined.

Remove the bowl from the mixer and fold once or twice with a spatula to ensure a smooth batter.

Divide batter evenly between prepared pans (about 380g per 8-inch pan for 3 layers). Bake 15–18 minutes, until the edges are just lightly golden and the center springs back when gently pressed. Cool completely before frosting.

Bring about 1 inch of water to a simmer in a medium saucepan.

Whisk egg whites, sugar, and salt together in the bowl of a stand mixer. Place the bowl over the simmering water (water should not touch the bowl) and whisk constantly until the mixture reaches 120–130°F and the sugar is fully dissolved.

Transfer the bowl to the stand mixer fitted with the whisk attachment. Beat on high speed for 5–7 minutes, until stiff peaks form and the bowl is just slightly warm to the touch.

Switch to the paddle attachment. Add the butter all at once and beat on medium speed until the buttercream comes together and looks smooth and silky. This can take several minutes and may look broken before it emulsifies.

Add the lemon curd and switch back to the whisk attachment. Beat on medium-high speed for 1–2 minutes until light, fluffy, and evenly combined.

Place one cooled cake layer on a cake board or serving plate.

Spread about 1 cup of buttercream evenly over the top.

Repeat with remaining layers, placing the final layer upside down for a flat top.

Apply a thin crumb coat over the cake and refrigerate for at least 30 minutes.

Frost with remaining buttercream and finish as desired. Serve at room temperature.