Preheat your oven to 300 degrees fahrenheit.
Butter the bottom and sides of a 9-inch springform pan. You can also line the bottom of the springform pan with parchment paper (on top of the butter) for easy removal.
Add the chocolate and butter to a large glass bowl and melt over a double-boiler, stirring until fully melted. Alternatively, you can slowly heat the chocolate and butter in 20-second increments in the microwave. Remove the bowl from the double-boiler (or microwave) and let cool to room temperature.
While the chocolate is cooling, add the egg whites to a large bowl and beat on medium-high until soft peaks form. Slowly add the coconut sugar while beating, about one tablespoon at a time, until fully incorporated. Set aside.
To the melted and now room temperature chocolate add the egg yolks and espresso powder and stir together. (*Please make sure the chocolate is cooled to room temperature, otherwise you'll end up with scrambled eggs in your chocolate).
Add 1/3 of the egg whites to the chocolate, then stir to combine and lighten the chocolate mixture. Gently fold in the remaining egg whites, trying not to deflate the mixture too much.
Pour the batter in the springform pan and bake for 45 minutes. The cake will rise to the top of the springform pan while baking, but will deflate once cooled.
Let the cake cool completely, then remove from the springform pan. Dust with cacao powder before serving.