Preheat your oven to 300 degrees Fahrenheit.
Prepare a 9-inch spring form pan by greasing it with butter and dusting the inside with cocoa powder.
In a large bowl, whip the egg whites and the cream of tartar on high speed with a hand mixer (use a stand mixer if you have one) until they reach stiff peaks. Set aside.
In a separate bowl, add the chocolate (chop it first), the butter and the espresso powder. Melt these in the microwave on 50% power for 30-second increments just until melted.
Stir the chocolate mixture well and set aside to cool.
In another bowl, whisk together the egg yolks and sugar until smooth. The mixture should be very pale.
Once the chocolate mixture has cooled, add the egg yolk and sugar mixture to the chocolate mixture and whisk them together well. Mix the salt in as well.
Switch to a silicone spatula and very gently fold in about one-third of the egg whites, just until the white streaks disappear.
Add another 1/3 of the egg whites and fold them in gently.
Add the last of the egg whites and continue folding gently until no more white streaks appear.
Pour the batter into the prepared cake pan and bake for 50-55 minutes at 300 degrees Fahrenheit.
Let the cake cool in the pan for about 15 minutes before releasing the sides of the pan and letting the cake cool on a wire rack until cooled completely.
Serve with a dusting of powdered sugar and some fresh berries.