Preheat oven to 350°F (176°C) and prepare two 8-inch cake pans with non-stick baking spray on the sides and parchment paper in the bottom.

Add the flour, sugar, baking soda, baking powder and salt to a large bowl and whisk together to combine.

Add the eggs, vegetable oil and lemon juice to the dry ingredients and whisk together to combine. Do not over mix.

Add the sour cream and whisk together until well combined. Do not over mix.

Divide the batter evenly between the prepared cake pans and bake for 22-27 minutes, or until a toothpick comes out with a few moist crumbs.

Remove the cakes from the oven and allow to cool for 2-3 minutes, then remove to a cooling rack to finish cooling.

Combine all the ingredients in a double boiler (or in a metal bowl over a pot of simmering water). Heat while whisking constantly until mixture thickens and reaches 160 degrees, or coats the back of a spoon.

Pour the lemon curd into a heat proof bowl, cover with clear wrap pressed onto the top of the curd to avoid a film developing, and refrigerate until cold.

To make the American buttercream, add the butter to a large mixer bowl and beat until smooth.

Add half of the powdered sugar and mix until well combined and smooth.

Add the lemon juice, lemon zest and vanilla extract and salt and mix until well combined and smooth.

Add the remaining powdered sugar and mix until well combined and smooth. Add heavy cream, as needed, to get the right consistency of frosting.

To make Swiss Meringue Buttercream:

place the sugar, salt and egg whites in a heat-safe mixer bowl over a pot of simmering water.

Dissolve Sugar - Whisk the mixture continuously until it becomes hot to the touch and the sugar is dissolved, about 3 full minutes.

Cool Meringue - Carefully remove the bowl from the heat and whisk on high for about 5 minutes, or until the meringue has cooled (I recommend using a stand mixer if you have one).

Add Butter - Using the paddle attachment mix in the butter a tablespoon at a time, beating until smooth.

Finish Buttercream - Once all the butter is in, beat in the mixture on high speed until it is thick and very smooth, 6-10 minutes. (If it separates the butter may be too cold, wrap heating pad around bowl or place in warm water bath).

Add Flavor - Decrease the speed to medium and gradually beat in the lemon juice, waiting until each addition is absorbed before adding more. Mix in vanilla. Set aside while preparing the cakes.

To put the cake together, use a large serrated knife to remove the domes from the top of the cakes, if needed, so that they’re flat. Next, cut each cake horizontally to make 4 layers of cake total.

To build the cake, place the first layer of cake on a cardboard cake circle or a serving platter. Pipe a dam of frosting around the outer edge of the cake, then fill it with about half of the lemon curd.

Add the second layer of cake on top, then spread about 1 cup of lemon buttercream evenly on top.

Add the third layer of cake, another dam of frosting and the remaining lemon curd. Top the cake with the final layer of cake.

Add a thin (crumb coat) of frosting to the outside of the cake, then place in the fridge to firm up for 1-2 hours. Once the frosting is firm and the cake is a little more steady, frost the outside of the cake.

Pipe swirls of buttercream on top of the cake with the remaining buttercream, if desired. Add lemon slices, if desired.

Refrigerate the cake until ready to serve. Cake is best served cool, but not cold. Store in an air tight cake carrier or well covered. Best if eaten within 3-4 days.