Break up the chocolate and put it on the stove in a “bain-marie”; when it begins to melt, add the butter in small pieces, stir carefully until a smooth cream forms and turn off the heat.
Mix the cornflour with the sugar in a bowl; then add them to the melted chocolate in the “bain-marie” and stir everything together well. Separate the eggs, putting the whites aside in a large bowl; mix the yolks with the vanilla seeds (slit the pod open lengthwise and scrape out the seeds with the flat tip of a knife) and the orange liqueur, then stir them into the chocolate mixture. Beat the egg whites, not to “stiff peaks” but slightly more liquid, and add them to the chocolate mixture; stir from bottom to top and when the mixture is smooth and homogenous, pour it into the cake tin, previously well-greased with butter. Bake in a preheated oven, at 180 °C, for 30 minutes. Let the cake cool down completely, open the “spring” of the cake tin and – taking care that it doesn’t break – arrange it on a plate and sprinkle with icing sugar. It’s absolutely delicious... and gluten free!