Pour boiling water into the cup and mix it with the cocoa powder. Do not leave any lump in it then set it aside and wait until it cools down a bit. Mix flour with the baking powder.
Beat egg whites until hard peaks form.
Pour sour cream into the lukewarm cocoa mixture. Mix to get an even, creamy texture.
Slice butter and put it into a mixing bowl to whisk until it becomes almost white and creamy.
Put the yolks one by one into the creamy butter. Add the stevia or sweetener of choice and keep whisking it.
When all of the yolks are in the creamy butter, turn the mixer to low speed and add the cocoa sour cream mixture.
Add the flour bit by bit too.
Mix each addition just until incorporated completely.
Carefully fold in the beaten egg whites.
Once it gets a homogeneous texture pour batter into the prepared tray.
Place it into the 350°F / 180°C preheated oven for 40 minutes to bake.
When it's baked transfer the cake onto a wire rack and let it cool down completely.
Slice room temperature butter and put it into a mixing bowl. Whisk it for about 5-10 minutes to get a light yellow, soft and creamy texture.
Put in stevia or sweetener of choice and cocoa powder then keep mixing until incorporated with the butter cream evenly.
When chocolate is melted take the bowl off the steam and let it cool a bit before folding it into the butter cream. It's important not to pour hot chocolate into the butter mixture as it can spoil the whole frosting. (Hot chocolate would melt the butter.)
Beat heavy cream in a different bowl then mix into the frosting too.
If the cake's top is not flat enough cut its top off.
The base has to be completely cool as it would make the frosting with the butter and cream runny which would be quite sad after all of this hard work.
Put some berries on top for decoration.
Keep cake in the fridge and take slices out 20 minutes before serving to allow it to warm up to room temperature so the creamy topping would taste as it should. Optionally we can place a slice into the microwave for 10 - 15 seconds if we haven't got time to waste.