Put all of the ingredients in a saucepan and cook it over medium-low heat whisking constantly.
Once the butter melts, the consistency of the mixture will be thin, don't increase the heat. Keep whisking until the mixture thickens for about 5 minutes.
When the mixture thickens, transfer it to a fine-mesh strainer set over and bowl and strain the mixture from any unnecessary lumps. This will give you a perfect and smooth curd.
Cover the bowl tightly with plastic wrap and let the mixture cool for about 20-30 minutes at room temperature, then transfer it to the fridge to completely set.
Preheat oven to 180c (350f) and prepare 3 6-inch baking pans with parchment paper.
In a large mixing bowl, add the soft butter, vanilla extract, granulated sugar, kosher salt, and lemon zest. Mix with an electric hand mixer until creamy and smooth for about 3 minutes.
Add the eggs and mix until they are well incorporated.
Gradually while mixing, add the oil and pour it slowly until the mixture is homogenous, then add the buttermilk and mix until well combined.
Add 1/2 of the flour, baking powder, and baking soda, and mix until almost combined. Then, add the other half of the flour and finish mixing with a rubber spatula until just combined and there are no lumps of flour visible.
Divide the mixture into 3 6-inch baking pans, I suggest using cake strips for even baking. Bake in a 180c (350f) preheated oven for about 25-30 minutes or until a toothpick comes out clean with a few moist crumbs.
Allow the cake layers to cool completely. Run a butter knife or a thin spatula around the edges of the cake to release it from the sides of the pan.
Invert and Tap: Place a wire rack or plate over the pan, hold them firmly, and flip to invert the pan. Lift Gently: Carefully lift one side of the pan, then remove it completely to slide your cake onto the surface.
Into a large mixing bowl add the heavy cream, powdered sugar, lemon zest, and vanilla extract, and mix with an electric hand mixer until stiff peaks form.
Add the room-temperature cream cheese and mix until smooth and creamy.
Pipe a small dollop of the frosting on the surface that you will frost on. Lay the first cake layer over the frosting dot.
Pipe a circle around the edges of the cake using the frosting. Then fill the circle with 2-3 tablespoons of the lemon curd and smooth it out with an offset spatula.
Top with the second cake layer and repeat the same process. Top with the third cake layer and frost the cake with more frosting. Smooth it out with a cake scraper to make it smooth.
Optional: make a drip design with the lemon curd and top it with two slices of lemon.