Preheat the oven to 350°F. Spray a loaf mold or 9x5-inch loaf pan with baking spray or grease it with flour and butter, then set it aside.
In a medium bowl, whisk the egg whites until fluffy. Set aside.
4 large eggs
In a separate bowl, beat the egg yolks, butter, sugar, and orange zest using a hand mixer until well combined.
1/2 cup salted butter, 4 large eggs, 1 cup (200g) granulated sugar, 1-2 Tablespoons orange zest
In another large bowl, combine the flour, baking powder, salt, sour cream, and orange juice. Mix until fully incorporated.
1 1/2 cups (212g) all-purpose flour, 2 teaspoons baking powder, 1/2 cup freshly squeezed orange juice, ⅓ cup sour cream, 1/2 teaspoon salt
Gradually add the butter and egg yolk mixture to the flour mixture, being careful not to overmix.
Finally, fold in the egg whites gently until just incorporated, maintaining the fluffiness of the mixture.
Pour the batter into the prepared cake mold, filling it about ⅔ full.
Bake in the preheated oven for 40 to 50 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for about 15 minutes before removing it.
Sprinkle powdered sugar on top and garnish with extra orange slices before serving.
Powdered sugar,