Melt 300 gr chocolate together with the vanilla using a bain-marie.Add the syrup, stir, add the cognac and stir again.
Put the crumbled sponge cake in a bowl with the walnuts, add the chocolate mixture and stir well.
Place the mixture in the fridge so that it tightens.
Put the mixture onto a plastic wrap, roll it into a cylinder, wrap it well and place in the fridge for a day.
Melt the rest of the chocolate (400 gr) in bain-marie together with the half spoonful of butter.
Remove the plastic wrap and cut the roll into slices about half a centimeter each. We leave them for a while in the freezer till well frozen.
Dip them into the chocolate (which we always keep over very low heat) using two forks.
Lay them out on baking paper and let them cool for a few hours (or put them back in the freezer for a while).
Wrap the cariocas in aluminum foil and keep them in the fridge.
