Preheat the oven to 300°F (150°C). Butter the bottom and sides of a 9-inch springform pan. For easier removal, you can also line the bottom with parchment paper.

In a large glass bowl, melt the chocolate and butter together over a double-boiler, stirring until smooth. Alternatively, heat the chocolate and butter in 20-second intervals in the microwave, stirring each time. Allow the mixture to cool to room temperature.

While the chocolate is cooling, add the egg whites to a large bowl and beat on medium-high speed until soft peaks form. Slowly add coconut sugar, one tablespoon at a time, until fully incorporated. Set aside.

Once the chocolate has cooled, add the egg yolks and espresso powder, stirring until combined.

Gently fold in ⅓ of the whipped egg whites into the chocolate mixture to lighten it. Then, gently fold in the remaining egg whites, ensuring the mixture stays airy without deflating.

Pour the batter into the prepared springform pan and bake for 45 minutes. The cake will rise slightly during baking and then deflate as it cools.

Let the cake cool completely in the pan. Once cooled, carefully remove it from the springform pan. Dust with cacao powder before serving.