First, melt the butter over very low heat, then let it cool slightly.

Place the eggs and sugar in the bowl of a stand mixer.

Beat with the whisk attachment at medium speed, or use electric beaters.

When the eggs are almost fully whipped, add the salt and grated lemon zest. Beat at high speed for 8 minutes.

Add the egg yolks one at a time. In another bowl, combine the flour, baking powder, and potato starch, then sift them together onto a sheet of parchment paper twice.

Reduce the mixer speed and add all the dry ingredients at once. Increase the speed to blend until the batter is thick and smooth, then turn off the mixer.

Scoop out a portion of the batter and transfer it to a bowl.

Add the melted butter and mix with a spatula. Add a little more batter to the bowl and mix until the mixture is uniform.

Pour the mixture back into the stand mixer bowl and fold gently from bottom to top.

Once the batter is smooth, transfer it to a greased and floured 8-inch (20 cm) cake pan.

Bake in a conventional oven at 375°F (190°C) for 40 minutes.

Remove the cake from the oven and let it cool.

Turn the cake out onto a wire rack, dust with powdered sugar, and serve your Torta Margherita.