Preheat the oven to 350° F/ 180° C.
In a small bowl, combine flaxseed meal with water and stir. Set aside for 5 minutes.
In another bowl, sift and combine coconut flour, cocoa powder, and baking powder. Whisk well.
In a large mixing bowl, beat together eggs and sweetener with an electric mixer for 3 to 4 minutes, until the mixture increases its volume and becomes pale.
Add oil and yogurt, and mix again shortly. Add the soaked flaxseed meal and mix to combine.
Using a rubber spatula, gradually add the dry ingredients. Fold them gently into the batter until completely absorbed.
Transfer the batter to a 9-inch heart-shaped pan (approx. 6 ⅓ cups or 1,5 l in volume), greased and lined with parchment paper.
Bake for about 40 minutes or until a toothpick inserted in the center comes out clean.
Let cool to room temperature before assembling the cake.