• Preheat the oven to 350°F and spray 3, 8-9” round cake pans with baking spray or line with parchment paper.
• In the bowl of a hand or stand mixer fitted with the paddle attachment add the 2 cup sugars, 3.5 cups all purpose flour, 2 tsp baking powder, 1 tsp kosher salt, 1/2 tsp baking soda, and 1 tbsp lemon zest.
• Mix on low speed until combined. Add in the 1 cup butter and mix for 30 seconds to one minute until the mixture looks sandy.
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  With the mixer on low speed add ¼ cup fresh lemon juice, and 1 cup buttermilk. Mix again for 30 seconds moving up from low to medium speed. Stop the mixer and scrape down the sides.
• Add the 2 eggs and egg white and mix on medium for 30 seconds.
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  Pour the batter evenly into the 3 pans and bake for 25 minutes. Cool in the pan for 10 minutes then flip onto a wire rack to cool completely.

Make the lemon curd.

• Separate the egg yolks from the whites carefully. Set the whites aside if you want to make the meringues to decorate. If you get any yellow into the whites the meringues wont whip so you need to be very careful.
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  Combine the 5 egg yolks, 3/4 cup sugar, lemon zest, and 1/2 cup freshly squeezed lemon juice in a small bowl or measuring cup and whisk to combine.
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  Pour it into a heavy bottomed pan over low- medium heat. Consistently stir the mixture for 10 minutes until it has thickened. You don't want to rush this part or raise the heat as it will cook the eggs.
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  The mixture is done when it coats the back of a spoon. If you take your finger and make a line through the curd on the back of the spoon and it stays in place, it's done.
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  Take the pan off the heat and immediately add the 6 tbsp cold butter. Stir until melted.
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  Strain the curd through a fine mesh strainer to make it smooth.
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  Pour the curd into a container and place a piece of plastic directly on top to prevent a film. Then cover and chill in the fridge for 1 hour.

Optional meringue decorations

• Preheat the oven to 200 F and line a sheet tray with parchment paper.
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  Combine the remaining 5 egg whites and ⅛ tsp cream of tartar in the bowl of an electric mixer like a hand mixer or stand mixer. Whip on medium speed until foamy, then slowly pour in the 1 ¼ cup sugar. Move to high speed and whip for up to 5 minutes for glossy stiff peaks. Do not overmix.
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  Add the meringue to a piping bag fitted with a large star tip and pipe meringues on the parchment paper. They can be close together as they won’t rise. Bake for 90 minutes, then shut off the oven and leave the meringues in for another hour. Remove from the oven and place in an airtight container if not using right away.

Make the lemon buttercream.

• In the bowl of a hand or stand mixer fitted with a paddle attachment, combine the 3 1/2 cups confectioners sugar and 1 cup softened butter. Mix on low speed working up to medium as the sugar combines with the butter. About 1-2 minutes.
• Stop the mixer and scrape down the sides. Turn the mixer back to low speed and add the 2 tsp lemon zest, 2 tsp of lemon juice, 1 tbsp milk, and the pinch of salt if using. Mix on medium for 3 minutes until fluffy.
• Level off the tops of the cake if needed. Fit a piping bag with a large round tip or fill a zip top bag with the frosting and snip off the end.
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  Place one layer of cake on a cake board. Pipe a line around the edges of the cake. This is called a dam, it helps keep the lemon curd filling inside the cake without spilling out. Fill the inside with lemon curd.
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  Repeat with the second cake layer, then top with the last layer of cake.
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  Frost the cake with the remaining lemon buttercream and decorate immediately with the meringues.