Preheat your oven to 350°F (177°C) for convection baking or 325°F (162°C) for conventional baking without a fan. Line the bottoms of two 8-inch (20 cm) round cake pans with parchment paper.

In a large mixing bowl or the bowl of a stand mixer fitted with a whisk attachment, whisk the egg whites with the granulated sugar on high speed. Continue until the mixture forms stiff peaks and is glossy.

Reduce the speed to medium and add the egg yolks, melted butter, and vanilla extract. Beat only until just combined to preserve the airy texture.

Sift the all-purpose flour and fine sea salt over the mixture. Gently fold the dry ingredients into the batter, then briefly beat on medium speed until no streaks of flour remain.

Divide the batter evenly between the prepared cake pans, approximately 17.5 ounces in each. Bake for 25 minutes, or until the cakes are golden, and the centers spring back when lightly pressed.

Once baked, run a knife around the edges of each pan to loosen the cakes. Allow them to cool for 15 minutes in the pans, then transfer them to a cooling rack or clean kitchen towel to cool completely. For a layered effect, use a serrated knife to slice each cake horizontally into halves or thirds.

In a small saucepan, combine water, sugar, and freshly squeezed lemon juice. Heat over medium-high until the mixture comes to a gentle boil. Reduce the heat to medium-low and simmer for 2–3 minutes. Remove from heat and pour into a heat-safe container. Let it cool fully before using it to moisten the cake layers.

In a small saucepan, whisk together the sugar, eggs, lemon juice, lemon zest, melted butter, and salt. Cook over medium-low heat, whisking constantly to prevent curdling, until the mixture thickens to a pudding-like consistency. This should take about 5 minutes.

Strain the curd through a fine-mesh sieve into a bowl to remove any lumps. Cover it with plastic wrap, ensuring it touches the surface to prevent a skin from forming, and let it cool to room temperature. Store any leftover curd in the refrigerator.

Sift the powdered sugar and salt into a mixing bowl or the bowl of a stand mixer. Add the cold heavy whipping cream and vanilla bean paste. Begin whipping on medium speed to avoid splattering, then increase to medium-high. Whisk until the cream holds stiff peaks, being cautious not to over-whip.

If the mixture becomes grainy, fold in a small amount of additional cream to restore smoothness.

Slice the cooled cakes into four thin layers. Place one layer on your serving surface and brush generously with the cooled lemon simple syrup. Spread a layer of Chantilly cream evenly across the top, then swirl in about ¼ of the prepared lemon curd.

Repeat with the remaining layers, stacking them neatly. For the final layer, brush it with simple syrup before placing it on top, sliced side down. Spread a generous layer of Chantilly cream over the top and finish with a swirl of lemon curd for a decorative touch.

Refrigerate the assembled cake for at least an hour or, ideally, overnight. Serve cold or slightly chilled, storing leftovers covered in the refrigerator.