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Pistachio Cake
Weigh eight eggs in their shells and allow an equal weight of caster-sugar and the weight of two eggs of potato flour and the same of pistachio nuts. Separate the yolks from the whites of the eggs and beat the yolks with the sugar; beat the whites of the eggs to a stiff snow and blanch and pound the nuts, stirring in gradually half of the potato flour, and when well beaten half of the whites of egg. When quite smooth add the remainder of the flour, the beaten whites of egg and then the pistachios. The potato flour should be sprinkled in very gradually or it will become lumpy. Pour this mixture into well-buttered cake-tins and bake in a slow oven. When done turn the cake out onto a piece of white paper and allow it to get cold; then mask it with lemon-flavored icing, smoothing it over evenly with the blade of a knife, and place in a cool oven for a minute or two to dry the icing. If desired the cake may be decorated with fancy designs in angelica, preserved cherries, etc. It should be kept in a cool place.
A weight-ratio sponge of the old European school: the eggs set the scale for everything else, so a kitchen scale is not optional here. The written method is slightly garbled in the original — the pounded nuts are folded in last, as the closing sentences confirm.
- Weighing by eggEight large eggs in the shell run about 1 lb / 450-480 g, so you need the same in caster sugar. The weight of two eggs is roughly 4 oz / 115 g each of potato flour and shelled pistachios. Weigh your own eggs rather than trusting these figures.
- Potato flourThis is potato starch (fécule), sold today as potato starch and found in Passover baking sections; the modern product labelled 'potato flour' is a different, coarser thing. It gives a very light, slightly springy crumb and no gluten structure at all, which is why the whipped whites carry the cake. Cornstarch is the nearest substitute but bakes drier.
- Order of mixingBeat yolks and sugar to a thick pale ribbon, whip the whites to firm peaks, then alternate as the recipe directs: half the starch sifted over the yolks, half the whites folded in, then the remaining starch and the remaining whites. The pounded pistachios go in at the very end so the batter is not deflated by working them through.
- Sprinkle, do not dumpThe warning about lumps is the real hinge. Potato starch clumps instantly on contact with wet batter; sift it over the surface in thin veils and fold with a spatula rather than stirring.
- Blanching the nutsPour boiling water over the shelled pistachios, leave one minute, drain and rub off the skins — this is what keeps the green colour bright. Pound or pulse them with a spoonful of the measured sugar so they grind rather than turn to paste.
- Oven temperatureA slow oven here means 300-325F (150-165C). A deep 9-inch tin will take 45-55 minutes; two shallower tins, 25-30. The cake is done when the top springs back and a skewer comes out clean.
- Drying the icingThe lemon icing is a simple water icing — confectioners' sugar with lemon juice, spread thin. The "cool oven for a minute or two" means about 200F (95C) with the door ajar, just long enough to set a matte skin. Any hotter and it slumps.
- YieldEnough for a single large cake serving 10-12, or two smaller ones. Angelica and glacé cherries are still sold, though angelica is now a specialist item.
Weighing eggs in the shell and matching them with sugar, flour and nuts is the arithmetic behind the whole family of European sponge cakes — the French quatre-quarts and the Genoese and Savoy sponges all work from equal or proportional weights rather than cup measures. Potato starch in place of wheat flour was valued for an especially fine, tender crumb. Pistachios were an expensive imported luxury, valued as much for the green colour they gave a pale sponge as for their flavour; the angelica and preserved cherries named at the end belong to the same decorative vocabulary, where a cake was expected to be masked smooth and ornamented before it left the kitchen.