Preheat the oven to 350°F (180°C) and grease a cake pan with butter.
In a large mixing bowl, combine the Erythritol with the almond and coconut flour, cacao powder, baking powder and soda, and salt.
In another mixing bowl, whisk the eggs with an electric mixer until they double in bulk.
Add the Greek yogurt, vanilla and melted butter and keep whisking to combine.
Add dry mixture to the wet mixture and whisk constantly with the electric mixer.
Finally, add the sparkling water to the batter.
Pour the batter into the prepared cake pan and bake in the preheated oven for 40-45 minutes, covering the top surface with baking paper or foil if needed.
Let the cake reach room temperature before removing it from the pan.
Optionally prepare my keto chocolate frosting while the cake cools, trust me it's worth it!