Preheat the oven to 300°F (150°C). Grease the bottom and sides of a 9-inch springform pan with butter, optionally lining the bottom with parchment paper for easier removal.

Combine the semisweet chocolate chips and butter in a large heatproof bowl. Melt over a double boiler, stirring continuously until smooth, or microwave in 20-second bursts. Remove from heat and allow to cool to room temperature.

In a separate large bowl, beat the egg whites on medium-high speed until soft peaks form. Gradually incorporate the coconut sugar approximately one tablespoon at a time while beating until fully integrated. Set aside.

Add the egg yolks and espresso powder to the cooled room-temperature chocolate mixture and stir until combined.

Stir one-third of the whipped egg whites into the chocolate mixture to lighten the texture. Gently fold in the remaining egg whites just until combined, taking care not to overmix and deflate the batter.

Transfer the batter into the prepared springform pan and bake for 45 minutes.

Allow the cake to cool completely before removing the sides of the springform pan. Dust the top with cacao powder before serving.