Preheat your oven to 180°C (350F). Line three 20 cm pans with parchment paper and grease with non-stick spray.

In a stand mixer or with a hand mixer, combine the flour, sugar, baking powder and salt. Mix until fully combined.

Gradually add the butter to the dry ingredients while mixing on low speed.

Add the lemon zest to the bowl.

Continue mixing until the butter is fully incorporated, and the mixture resembles coarse, moist sand.

Slowly mix in the egg whites until just combined. Then, add the buttermilk in two parts, mixing on low speed after each addition.

Add the vegetable oil and vanilla extract, mixing on low speed until fully combined.

Scrape down the sides of the bowl with a spatula, then mix on medium speed for about a minute to ensure the batter is smooth and the texture is light.

Evenly divide the batter between your prepared pans. Use a kitchen scale to ensure each pan has the same amount for even baking.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.

Allow the cakes to cool for 10 minutes and remove from the pans.

Use a serrated knife to level the tops of the layers when you’re ready to assemble your cake.

Combine the sugar and water in a medium pot and stir over medium heat until the sugar dissolves. Once the mixture starts to boil, stop stirring and attach a thermometer to the pot. Cook the syrup until it reaches 115°C (240°F).

Separate the egg whites one by one in a separate bowl, then transfer them into the mixing bowl. If any yolk touches the egg whites, discard the egg and thoroughly wash the bowl before starting with a new egg. It’s crucial to keep the egg whites free from fats. To ensure the bowl is spotless, I like to wipe it with vinegar to remove any potential fat residue.

Add cream of tartar or lemon juice to the egg whites and attach the whisk to the mixer.

When the sugar reaches 110°C (230°F), begin whipping the egg whites on medium-high speed. The goal is to reach soft peaks by the time the syrup hits 115°C (240°F).

With the mixer running, slowly pour the hot sugar syrup into the egg whites in a steady stream. Continue whipping on high speed until stiff peaks form and the bowl feels cool to the touch. The meringue should be smooth and glossy, not clumpy or dry.

With the mixer still running, gradually add room-temperature butter in small chunks. Keep mixing until all the butter is incorporated, and the buttercream becomes light and smooth.

If the buttercream looks runny at the end of this process, chill the bowl in the refrigerator for a few minutes, then whip it again until the desired consistency is reached.

In a medium heatproof mixing bowl set on top of a dampened towel (this serves as a stable base), stir together sugar, cornstarch, and salt. Whisk in egg yolks until mixture is pale yellow, smooth, and fluffy, about 1 minute.

In a pot on medium heat, combine milk and lemon zest. Bring to a bare simmer.

Remove the milk from heat. While whisking continuously, slowly pour the milk into the egg yolk mixture in a thin stream until all of it has been added.

Return the mixture back to the pot. Cook over medium heat, whisking constantly until pastry cream begins to thicken, about 2 minutes. Once it thickens, continue to whisk, pausing every few seconds to check for bubbles. When it begins to bubble, set a timer and continue whisking for 1 minute.

Remove fromt he heat and add the butter. Whisk until melted and thoroughly combined.

Immediately place plastic wrap or buttered parchment paper directly on the surface of the cream to prevent a skin from forming.

Refrigerate until cold.

In a large saucepan, combine the sugar, water and lemon juice. Bring to a boil. Reduce to a simmer (med-low) and add lemon slices in a single layer.

Simmer over medium-low (uncovered) for 15 minutes, gently flipping once or twice during the cooking process.

Transfer the lemon slices to a wire cooling rack or waxed paper and let cool.

To put this together I have created a ring of buttercream around the first cake layer. I then added half the pastry cream and added a couple tablespoons of lemon curd. At this point try to distribute it evenly.

Add the second cake layer and repeat the process.

Cover with the last cake layer.

Once the cake is stacked, place your hands around the cake and push in place any layers that are not straight.

Apply a thin layer of buttercream around the cake to coat any cake crumbs.

Refrigerate for a couple of hours so that the butter in the buttercream stiffens and the cake gains some structure.

Once the cake has chilled it is now ready for the final buttercream coating and decorations.

In my case i have added some extra lemon curd on top and piped some rosettes.

Finally decorate with the candied lemon slices.