Preheat oven to 175°C. Prepare a springform cake tin; Generously butter and flour the inside and wrap the outside of the tin in alfoil. Place tin in a large baking dish.
Place egg whites and cream of tartar into the Thermomix bowl and whip 4 min/37°C/Butterfly/speed 3. Then set Thermomix to 5 min/Butterfly/speed 3 and gradually add 50g sugar one teaspoon at a time. Remove the meringue from bowl and set aside.
Place chocolate and butter into the Thermomix bowl and melt 3 min/50/speed 2. Set aside and cool slightly.
Place egg yolks and 100g of sugar into the Thermomix bowl and whip 2 min/Butterfly/speed 3. Add brandy, vanilla, salt and the chocolate mixture and combine 10 sec/speed 3. Remove Butterfly. Add the whipped egg whites a little at a time, folding in carefully until just combined.
Pour mixture into the prepared tin and place tin in a large baking dish.
Pour boiling water into the baking dish until it reaches halfway up the tin. Carefully place in the oven. Bake for 50 minutes. Remove from oven and leave cake in tin until it's completely cool.
Dust cake with cocoa. Garnish with plenty of whipped cream if desired and dust with more cocoa. Cake is best eaten at room temperature.