Prepare a large bowl and melt the chocolate together with butter using the double boiler technique. To do this, place the bowl over a pot of simmering water. Make sure the water doesn't touch the bottom of the bowl. Stir the mixture until the chocolate and butter melt and combine evenly. Afterward, let the mixture sit until it's not too hot.

In another bowl, mix together the egg yolks, cocoa powder, vanilla extract, espresso powder, and heavy cream. Stir until well combined and the mixture is homogeneous.

Gradually pour the melted chocolate mixture (from step 1) into the bowl with the egg mixture (from step 2). Gently stir until all the ingredients become smooth.

In a separate bowl, use a mixer to beat the egg whites and cream of tartar. Then, gradually add the granulated sugar while continuing to mix at high speed until the egg whites are soft peaks.

Fold a small portion of the egg white mixture (from step 4) into the chocolate mixture (from step 3). Stir until evenly mixed.

Slowly incorporate the remaining egg white mixture into the chocolate mixture, using a folding technique (gently folding the batter from bottom to top) to ensure the batter remains light and airy.

Pour the batter into a baking pan lined with parchment paper.

Bake the cake in a preheated oven at 180°C (350°F) for 25 minutes or until the cake is cooked. Use a toothpick or skewer to check the cake's doneness. If the toothpick comes out clean, the cake is ready.

Remove the cake from the pan and decorate it with icing sugar.

Slice the cake and get it ready for serving.