Separate the egg whites from the yolks. Set aside the egg whites for later.
Mix the flour and baking powder together by passing them through a sieve 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 stevia, eggs yolk, and mix until coconut sugar is dissolved. Add the natural orange color (optional). Add the flour and baking powder, 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 mix 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 add 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.