1 ½ cups all-purpose flour
½ teaspoon baking powder
½ teaspoon salt
4 eggs, separated
¾ cup plus 3 tablespoons superfine sugar, divided
2 sticks unsalted butter at room temperature
1 tablespoon lemon juice
2 teaspoons grated lemon zest
Preheat the oven to 350 F.
Grease a 9-by-5 loaf pan or Bundt pan with nonstick cooking spray.
Sift the flour, baking powder and salt together into a bowl, then repeat twice more
In a stand mixer, whisk the egg whites until they hold soft peaks, then add ½ cup of the sugar gradually, beating after each addition. Gently transfer the egg white mixture to a separate bowl, and clean out the mixer bowl.
Fit the stand mixer with a paddle attachment, and add the butter to the bowl. Beat for 2 minutes, then add the lemon juice, the zest and the remaining sugar. Beat until fluffy, then add the egg yolks and beat a final 1-2 minutes, scraping down the sides regularly to ensure everything is incorporated.
Remove the bowl from the stand mixer, and fold in the egg white mixture until just incorporated, then add the flour mixture a little at a time, folding after each addition, again until just incorporated.
Transfer the mixture to the loaf pan, and bake 35-45 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool at least 10 minutes before turning out onto a wire rack to cool completely.