In a saucepan, whisk together fresh lemon juice, egg yolks, granulated sugar, salt, and lemon zest over medium-low heat. Stir continuously for about 20–25 minutes until the mixture thickens and coats the back of a spoon. Remove from heat and stir in cold unsalted butter until melted. Cool to room temperature.

Preheat your oven to 350°F (175°C). In a mixing bowl, whisk flour, baking powder, baking soda, and salt. In another bowl, cream softened butter and sugar until fluffy, then beat in eggs and vanilla. Gradually add dry ingredients alternately with buttermilk, folding in lemon zest.

Grease and line a 9x9 inch baking pan with parchment paper. Pour batter into the pan and smooth the top. Bake for 35–40 minutes, or until a toothpick comes out clean. Allow the cake to cool completely in the pan on a wire rack.

Beat softened unsalted butter with an electric mixer until light and fluffy. Add cream cheese and continue mixing. Gradually sift in powdered sugar, then add lemon zest. Beat until smooth and creamy.

Once the cake is cooled, invert it onto a serving plate. Poke holes in the top using a skewer. Spoon lemon curd into the holes. Frost the top with cream cheese frosting and swirl additional lemon curd on top. Add lemon zest or slices for decoration if desired.