Prepare your station.
Preheat your oven to 350°F / 175°C. Thoroughly butter and flour one 10-inch springform pan or two 9-inch round cake pans.
Set aside.
Warm the eggs.
In the large bowl of a stand mixer (or a large heatproof bowl), combine the eggs, sugar, and grated lemon rind.
Place this bowl over a saucepan of gently simmering water (a bain-marie), ensuring the bottom of the bowl doesn't touch the water.
Whisk constantly until the sugar has completely dissolved and the mixture is warm to the touch.
Whip to ribbon stage.
Transfer the bowl to your mixer fitted with the whisk attachment.
Beat on high speed for 8-10 minutes. The mixture will become pale, creamy, and triple in volume. You'll know it's ready when you lift the whisk and the batter falls back in a thick, continuous ribbon that sits on the surface for a few moments before dissolving.
Fold in the flour. Sift the flour over the egg foam in three separate additions.
Using a large spatula, gently fold the flour into the batter with each addition, scraping from the bottom up. Use the minimum number of strokes necessary to avoid deflating the air you worked so hard to create.
Incorporate the butter. Gently pour the cooled, melted butter down the side of the bowl, leaving any milky sediment behind.
Fold the butter in with light, swift strokes just until it is incorporated.
Do not overmix.
Bake to perfection. Immediately pour the batter into your prepared pan(s) and gently smooth the top.
Bake for 45-50 minutes, or until the cake is a deep golden brown, has started to pull away from the sides of the pan, and a skewer inserted into the center comes out clean.
Cool completely.
Let the cake cool in the pan for 10 minutes before carefully inverting it onto a wire rack to cool completely. The cake must be fully cooled before slicing or assembling.