Preheat the oven to 160 Celsius (320 degrees Fahrenheit). Crack the eggs and separate the egg whites from the yolks. Set aside the egg whites for later.

Mix the flour (or almond flour if you are gluten free) and baking powder together by passing them through a sieve a couple of times. 

Grate the orange zest from the orange. Squeeze the orange using a hand juicer (or any other method you have at your disposal) and pass it through a sieve.

Mix the orange juice, zest and coconut sugar in a bowl with a whisk. Add your stevia sweetener, yolk from the eggs, and mix until coconut sugar is dissolved. Add the natural orange color (optional). Add the flour and baking powder and mix with a whisk or with a mixer on medium speed for 2 minutes. Add butter to the mix (the butter should be at room temperature and soft. If not: melt it in a microwave) 

Take the egg whites in the bowl and use a whisk to get stiff peaks. Use the hand mixer to beat the eggs if you like. This process will take 2-3 minutes when using a whisk. Add the egg whites to the mix.

Mix the ingredients well; do not beat much, just enough to mix the egg whites with the cake mix. Pour the batter in the well-greased baking tin. 

Place in the preheated oven with a water bath and bake for 50-55 minutes or until the toothpick comes out clean (the water bath adds moisture to the oven and provides a more even, slower heat source than the direct heat of an oven).

Allow the cake to cool in a baking tin, garnish with the leftover orange zest. Serve warm.