In a small saucepan, whisk together lemon juice, sugar, egg yolks, lemon zest, and a pinch of salt. Cook over medium-low heat, stirring constantly. Once the mixture is warm, add butter and continue stirring until it thickens (about 5–7 minutes). Do not let it boil. When the curd coats the back of a spoon, remove from heat. Strain if needed. Cool completely and refrigerate for at least 1 hour.

Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans. Line the bottoms with parchment paper.

Sift cake flour with baking powder and salt in one bowl. In a separate bowl, beat softened butter with sugar until pale and fluffy (3–5 minutes). Mix in lemon zest, vanilla extract, and oil.

Add the flour mixture in three parts, alternating with buttermilk, starting and ending with the dry ingredients. In another bowl, whip egg whites to soft peaks and gently fold them into the batter in three additions.

Divide the batter evenly into the prepared pans. Smooth the tops and bake for 25–30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

Beat cold mascarpone with powdered sugar until smooth. Slowly add lemon juice and vanilla. In a separate bowl, whip cream until stiff peaks form, then fold it into the mascarpone mixture. Chill for 15 minutes before using.

Once cakes are cool, level if needed. Place the first layer on a serving plate. Spread a thin layer of mascarpone frosting, followed by 2–3 tablespoons of lemon curd. Repeat with the next two layers.

Frost the top and sides of the cake with the remaining mascarpone frosting. Chill for at least 30 minutes before serving. Optional: garnish with lemon slices or zest.