Preheat oven to 160ºC (320ºF). Grease with butter and line with parchment paper a 9-inch (22cm) round springform cake tin. Set aside.
Prepare a double boiler with a large heat-proof mixing bowl over a pot of simmering water. NOTE: make sure the bowl is big enough as it's the bowl for the entire cake.
Add broken-up chocolate and butter to the bowl and gently melt together over low heat until completely melted and smooth.
Remove from heat and allow to cool slightly whilst you separate the eggs.
Separate the eggs and place the egg whites into a large mixing bowl or the bowl of a stand mixer.
Add yolks into a separate small bowl.
When the chocolate mixture has cooled slightly and is just warm to the touch when you dip your finger in, add egg yolks and vanilla, and whisk until well combined.
Sift over cocoa powder then add the coffee powder. Whisk until well combined and smooth. Set aside.
To whip the egg whites, use an electric hand beater or stand mixer with the whisk attached, and beat for 20 seconds until foamy.
Gradually pour in sugar, one tablespoon at a time, whilst beating on medium-high speed, beat for 30 seconds between each addition of sugar. Scrape down the sides if any sugar is stuck to the sides of the bowl. Continue to beat egg whites until stiff peaks form, about 3 minutes in total.
To test if the egg whites are done, stop mixing, lift the beaters out of the bowl and the egg whites should hold their shape and not fall back down into the bowl.
Stir the chocolate mixture again, if it’s turned too thick, mix it well to loosen it up.
Add 1/3 of whipped egg whites to the chocolate mixture, and gently fold with a spatula until just combined. Take this step slowly and gently fold to avoid deflating the egg whites too much.
Add another 1/3 of the whites and fold through until just combined, some white streaks will still be visible.
Add the remaining 1/3 of the whites and fold until fully incorporated with no white streaks remaining. Do not over-mix, just until you see no white streaks left.
Pour batter into the lined springform pan, smoothing out the top with a spatula.
Bake for 30-35 minutes or until a toothpick inserted into the middle comes out with just a few moist crumbs still, the middle no longer jiggles and cracks have formed on the surface. Do not to over-bake as it will dry out in the middle.
Remove from oven and allow to cool in pan for 15 minutes.
Gently release the latch on the springform tin and lift out the cake still on the base of the tin.
Cool it on the base for a further 10-15 minutes until it's easier to slide it off. It's ok if more cracks appear when you're removing it.
Transfer the cake to a wire rack to cool completely. You can also cool it in the refrigerator to fasten up the process.
Serve warm or cool with scoops of ice cream or dolloped cream if desired.
Store in a sealed airtight container, at room temperature for 1-2 days then keep it refrigerated for a further 2-3 days.