Preparation Steps

• Preheat the oven to 340°F (171°C). Line three 8-inch round cake pans with parchment and lightly grease the sides; dust with flour or use baking spray.
• In a measuring cup, stir together the milk and lemon juice; set aside for 10 minutes to gently thicken (this makes a quick lemon “buttermilk”).
• In a large bowl, add the sugar and lemon zest. Rub them together with your fingertips for 30–60 seconds until the sugar is fragrant and slightly damp.
• Whisk in the eggs until the mixture looks paler and lightly foamy, about 1 minute. Slowly stream in the oil while whisking. Whisk in the sour cream, vanilla, and lemon oil (if using) until smooth.
• In a separate bowl, whisk together the flour, baking powder, and salt. Add half of the dry mix to the wet ingredients and fold with a spatula just until combined. Stir in all of the lemon-milk mixture, then fold in the remaining dry ingredients until no dry streaks remain; avoid overmixing.
• Divide the batter evenly among the pans (about 450 g per pan). Smooth the tops and tap each pan once on the counter to release large air bubbles.
• Bake for 23–27 minutes, rotating pans front-to-back once midway. The cakes are done when the centers spring back and a toothpick comes out with a few moist crumbs.
• Cool the cakes in their pans on a rack for 10 minutes, then turn out, peel off parchment, and cool completely. For the cleanest layers, wrap and chill the cakes for 30 minutes before frosting.
• Make the frosting: In the bowl of a stand mixer fitted with the whisk, combine mascarpone, powdered sugar, cream of tartar, and a pinch of salt. Mix on low just to loosen, 15–20 seconds. With the mixer running on medium-low, slowly stream in the cold cream. Increase to medium-high and whip until medium peaks form, 1–2 minutes. Fold in lemon zest and vanilla; whip 5–10 seconds more to reach firm peaks. Do not overwhip.
• Level any domed tops. Place a small swipe of frosting on a cake board and set down the first layer. Spread about 3/4 cup frosting evenly to the edge. Repeat with the second layer and more frosting, then top with the final layer upside down for a flat surface.
• Apply a thin crumb coat over the top and sides with an offset spatula. Chill 10–15 minutes to set. Add the remaining frosting, swooping it smoothly around the sides and on top. Garnish with extra lemon zest curls if desired, slice, and serve.