Preheat the oven to 325°F.

Butter two 6” cake pans and line the bottom of each pan with a piece of round parchment paper cut to fit.

In a small mixing bowl, sift together flour and baking powder. Add the kosher salt, and mix with a whisk to distribute evenly. Set aside.

Rub sugar and lemon zest together in the bowl of a stand mixer. Add egg and vanilla to the sugar mixture and beat until light and creamy, about 2 minutes. Add the flour mixture and stir until just combined.

Microwave the milk and butter in a heatproof measuring cup or bowl for 1 minute, stir to ensure the butter melts completely. Add the hot milk mixture to the stand mixer bowl while stirring to combine. Scrape the bowl with a spatula to ensure thorough mixing.

Divide cake batter evenly between the 2 prepared cake pans, about 290 g of batter per pan.

Bake in the middle oven rack for 25 - 27 minutes until a toothpick inserted in the middle of the cake comes out clean.

Remove from the oven and allow to sit until cool to the touch. Run a butter knife along the edge to loosen the cakes if necessary and remove them from the pan. Peel off parchment paper at the bottom and allow to cool completely on a wire rack.

Add sugar, a pinch of kosher salt, lemon zest, lemon juice, and an egg to a small saucepan, whisk together over low heat until the mixture is thoroughly mixed.

Add the cubed butter to the saucepan and whisk until the butter melts completely. Continue to cook the mixture while stirring continuously until it thickens and coats the back of a spoon. This will take 3 - 4 minutes.

Strain the curd through a fine mesh sieve into a clean bowl, stir in a splash of vanilla. Let cool completely and refrigerate uncovered until ready to use.

In a large mixing bowl or a stand mixer bowl, cream butter and cream cheese together until thoroughly mixed and creamy.

Add vanilla extract, lemon juice and lemon zest, beat to combine.

Add powdered sugar and stir until incorporated, gradually increase the mixer speed and beat until fluffy.

Fill a pastry bag fitted with a large round tip (size 807) with some of the frosting and set aside.

Line a cake stand or plate with a few strips of parchment paper and place 1 layer of cake on top.

Add ⅓ cup of frosting on top of the cake and spread it out evenly with a spatula. Crumb coat the side of the cake with some additional frosting. Pipe a border around the edge of the cake layer using the previously prepared frosting in the pastry bag.

Add ⅓ cup of lemon curd filling within the frosting border and place the cake in the refrigerator to allow the frosting and filling to firm up before adding the second layer of cake on top, about 30 minutes.

Place the second cake layer on top of the refrigerated cake layer, and spread ⅓ cup of frosting on top. Crumb coat the side of the second cake layer and fill in the gap between the two layers with some additional frosting. Refrigerate for another 30 minutes to allow the crumb coat to firm up before finishing decorating the cake.

Once the crumb coat is set, take the cake out of the refrigerator and add a thin layer of frosting on the side and top of the cake. Smooth out the frosting with an offset spatula.

Add the remaining frosting to a pastry bag fitted with a star tip (1M), pipe a border around the edge. Add the remaining lemon curd within the border and spread it out carefully with the back of a spoon.

Refrigerate for 2 hours to let the frosting set before serving.