Lemon Curd:

Using a double boiler, add lemon juice, lemon zest, eggs, yolk, and sugar to the top of the double boiler and whisk constantly until it thickens, about 10 minutes.
Remove from heat and add in butter, stirring until combined.
Pour into a glass jar and cover. Let cool for 15 minutes, then refrigerate overnight or until it sets.
Cake:

Preheat oven to 350°F (175°C).
Beat butter and sugar until smooth. Add in eggs one at a time, mixing well.
Add in lemon zest, buttermilk, and vanilla extract. Gradually add flour, baking powder, baking soda, and salt until fully incorporated.
Pour batter into a greased 9×9 baking dish lined with parchment paper.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Let cool completely.
Frosting:

While the cake cools, beat together butter and cream cheese until smooth.
Add in powdered sugar and vanilla extract, beating until smooth.
Add heavy cream and combine until the desired consistency is reached.
Assemble:

Remove cake from the pan. Poke holes into the cake and fill with lemon curd.
Once fully cooled, frost the cake and add dollops of lemon curd on top of the frosting. Enjoy!