Preheat oven to 180 degrees Celsius. Grease and line a cake tin and set aside.

In a small saucepan over a medium heat, combine the butter and chocolate, stirring constantly, until completely melted. Whisk to combine completely, remove from the heat and set aside.

Separate 4 of the 6 eggs, and place the egg whites in a medium sized bowl and the egg yolks in a separate bowl. To the yolks, add the vanilla extract, cocoa powder, salt, the remaining 2 eggs and half of the sugar, whisking well to combine.

Slowly add the cooled chocolate mixture into the yolk mixture, whisking well, and set aside once combined completely.

Use an electric whisk on a medium to high speed to beat the egg whites until light and frothy. Gradually add the remaining sugar and beat until the mixture has firm peaks and goes glossy.

Carefully fold the egg whites into the chocolate mixture, folding with a spatula until just incorporated. Pour the batter into the pre prepared cake tin and smooth the top with a spatula.

Place in the oven and bake until the top is slightly cracked, and the middle no longer moves – around 50-55 minutes. Transfer the cake to a cooling rack and allow to cool completely.

Dust the cake with cocoa powder and top with your desired topping. I placed little fudge pieces on the top along with chocolate covered espresso beans. You could put berries on the top, icing sugar or whipped cream, depending on your preference.