Preheat oven to 180 degrees C. Grease a 20cm bundt tin, alternatively a square (or round) cake tin. In a bowl over simmering water, melt the butter and chocolate over a low heat, stirring as necessary. Once melted add in the dark rum and stir together.
In a separate bowl, beat the egg whites with 1/2 teaspoon caster sugar and pinch of salt until stiff peaks form.
In larger bowl beat the egg yolks and whole egg with the sugar until it’s light and fluffy.
To this mixture add the chocolate and butter and mix well. Fold in the flour using a whisk to break up any lumps.
After the flour has been thoroughly mixed in, fold in the egg whites in two amounts. When this is well mixed, pour into the cake tin and bake for approx 25 to 30 minutes or until a skewer inserted in the cake comes out clean.
Remove from the tin and allow to cool then dust with cocoa powder. For decoration, melt any extra truffles and drizzle over the truffle cake.