Dissolve the yeast in water which is barely warm and add to the flour in a bowl, working it in until you get a firm dough (that comes away easily from the sides of the bowl). Cover and leave in a dry place to prove for an hour.

When doubled in size, mix in the egg yolks, sugar, 2/3 of the lard, a pinch of salt and the finely-grated orange peel, working it all in your fingers to blend the ingredients thoroughly.

With the lard left over, grease a deep, rectangular cake tin and press the dough in, making sure that it is of even thickness all over (about an inch). Be careful here, because herein lies the secret of success. Leave it for 2 hours to rise again.

Bake in a hot oven for half-an-hour. Turn out and dust with vanilla sugar.