Combine eggs, salt, and vanilla. Beat on high for 2 minutes.
Add sugar and beat on high for a few minutes until the mixture is fluffy and lighter in color. Add oil, lemon juice, and zest. Mix on low just to combine.
In a separate bowl, add flour and baking powder and mix. Gradually add the flour mixture to the wet ingredients and mix on the lowest setting (I use a hand whisk to avoid over-mixing).
Add milk and mix on low only to combine.
Divide batter into 2 greased 8-inch pans. Preheat the oven to 165°C (329°F). Bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Combine lemon juice, eggs, and sugar in a saucepan.
Mix well and cook on medium heat. Stir continuously until the mixture thickens, about 5 to 7 minutes.
Remove from heat and add cold butter and lemon zest. Stir until butter is melted and fully incorporated.
Let the curd cool at room temperature. Cover it to prevent a crust from forming. Then place it in the fridge (better to make this in advance and fill the cake when it's cold).
For the Buttercream
In a clean and grease-free bowl, combine egg whites, sugar, and salt.
Place the bowl over a double boiler on low to medium heat without the bottom of the bowl touching the water. Stir constantly until the sugar dissolves completely.
Remove from heat and beat the mixture on high speed until stiff peaks form. (Note: Liquid egg whites won’t form stiff peaks but will still work.)
Let the meringue cool to room temperature before proceeding.
With the mixer on low speed, add the butter a small piece at a time until fully incorporated. Increase the mixer’s speed and whip until the buttercream is smooth and fluffy. (If it looks curdled, keep whipping—it will come together.)
Add lemon juice and lemon zest, when grating the zest ,make sure you don't use the white part as it is bitter.
Mix everything together on medium for a few minutes. If you want the smoothest buttercream texture, set your mixer on the lowest speed and let it run for 10 minutes. This will remove air pockets.
Video
Notes
Note: the time mentioned does not include assembling and decorating the cake.
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