Preheat your oven to 365°F (185°C). Prepare two 8.7-inch (22 cm) round cake pans by lining the bottoms with parchment paper and greasing the parchment paper and the sides of the baking pan with butter for easy removal.
Separate egg yolks from the whites. In a separate bowl, beat the egg whites with a pinch of salt until well combined and slightly foamy. Add 1 teaspoon of the sweetener and continue whisking until very firm.
In a mixing bowl, add the rest of the sweetener to the egg yolks and whisk until the volume has increased at least 2 times and the mixture turns light yellow.
To the beaten eggs, add orange zest and almond flour. Mix well until everything is fully incorporated.
Transfer the whisked egg whites to the bowl with the egg yolk mixture and stir lightly to combine.
Divide the batter evenly between the two prepared cake pans. If you only have one baking pan, skip this step and bake all the batter at once – you’ll be able to cut the cake layer into two after baking.
Place the pans in the preheated oven and bake for about 15-20 minutes or until a toothpick inserted into the center of the cakes comes out clean. If baking all in one pan, the baking time will increase to 20-30 minutes.
Once the cakes are done baking, remove them from the oven and take off the sides of the baking pan. Then, turn the cakes upside down, remove the bottom plates of the baking pans and the parchment paper carefully. Place the sheets of the parchment paper that you just removed from the cakes onto the kitchen counter, the greasy side up, and transfer the cakes onto the sheet (basically, you’ll end up with the parchment paper placed on the opposite side of the cake). Let it cool for about 10 minutes. Then turn the cakes upside down again and remove the parchment paper. (Only when using one cake pan) Place the baked cake layer onto a flat and even surface and take a serrated bread knife. By holding the cake layer in place with your hand on top, cut the cake layer in half horizontally.
You can either start assembling the cake right away, store the cake layers in the fridge, or even freeze to ensure you have the cake layers at hand when you’re ready to finish your cake.