Preheat the oven to 180° Celsius

Place the butter into a bain-marie and leave to melt

Add the chocolate and cacao and leave to melt, stirring occasionally

When smooth, remove from the heat and set aside to cool slightly

Place the egg yolks into a medium sized mixing bowl and whisk until broken up

Add the fructose and whisk until just combined

Pour into the chocolate and whisk to mix

Add the flour and fold in until combined

Place the egg whites into the bowl of a stand mixer

Whisk until frothy and then add the cream of tartar

Continue whisking to soft peak stage

Add a third to the chocolate mixture and stir gently to combine

Then fold in the rest of the egg whites as gently as possible

When completely combined pour the batter into a lined spring-form tin

Bake for 45 minutes and remove from the oven

Leave to cool completely in the tin before removing