Whisk together 3 eggs, ¾ cup sugar, ½ cup lemon juice, and 1 tbsp zest in a saucepan. Add 4 tbsp butter and cook over medium heat, stirring constantly, until thick enough to coat a spoon, 5 to 7 minutes. Strain and chill with plastic wrap on the surface.

Preheat oven to 350°F (175°C). Grease two 8-inch cake pans and line with parchment. Whisk flour, baking powder, baking soda, and salt. Beat ½ cup butter and 1 cup sugar until fluffy, then beat in 2 eggs, 2 tbsp zest, and vanilla. Alternate adding dry ingredients with buttermilk and oil, mixing just until combined.

Divide batter between pans and bake 25 to 30 minutes, until golden and a toothpick comes out clean. Cool in pans 10 minutes, then turn out onto a rack to cool completely.

Beat cream cheese and ½ cup butter until smooth. Add powdered sugar in two batches, then vanilla and optional zest, and beat until fluffy, about 2 minutes.

Place one cake layer on a plate, spread chilled curd on top leaving a border, and top with the second layer. Spread a thin crumb coat of frosting over the whole cake and chill 20 minutes.

Frost the cake with the remaining frosting, then spoon a few tablespoons of curd on top and swirl with a toothpick. Scatter lemon zest over the top and serve.