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Portuguese Cake
Beat in a basin until frothy the yolks of six eggs, and half a pound of caster-sugar; stir a quarter of a pound of blanched almonds with the white of an egg to prevent oiling, rub them through a sieve into a kitchen basin, (ground almonds will do as well) and mix them with one pinch of grated orange-peel (to give zest) and the juice of three or four oranges. When the yolks of eggs and sugar are frothy, mix in gradually a quarter of a pound of potato-flour and the whites of six well-beaten eggs. Butter and paper a baking-sheet, pour the mixture on this about one inch thick, and bake in a slow oven. When the cake is done, allow it to cool, trim the surface, and mask first with a thin layer of apricot marmalade, and then with a layer of orange-flavored icing.
A flourless sheet torte thickened with potato starch and ground almonds, baked thin and finished with jam and icing. The whole thing hinges on how you fold the whites in.
- Fold, do not stirThe batter has no gluten and no chemical leavening; the six beaten whites are the only lift. Fold them in with the potato flour in alternating additions, cutting through with a spatula until just combined, and get the pan into the oven at once.
- Slow ovenRead that as 300-325F. A thin egg-foam sheet an inch thick will be done in roughly 25-35 minutes; it is ready when the surface springs back and the edges pull from the paper.
- Potato-flourPotato starch, sold that way today, not potato flour made from whole dried potato. They are not interchangeable: the starch gives the light, slightly springy crumb this cake wants.
- Caster-sugarSuperfine sugar, half a pound is about 1 cup plus 2 tablespoons. Granulated will work if you beat the yolks long enough to dissolve it fully.
- The almondsA quarter pound is about three-quarters of a cup of whole blanched almonds, or roughly a cup to a generous cup once ground. The egg white keeps them from turning to paste and oiling in a mortar or processor; with bought almond flour, skip that step and skip the sieve (the recipe itself allows ground almonds).
- The orange juice is a lotThree or four oranges is roughly three-quarters to one and a third cups of liquid, enough at the upper end to slacken the batter and knock the foam down. If you want to hedge, use a quarter to a third of a cup and lean on the grated zest; the juice is there for flavor rather than moisture.
- Butter and paperUse a rimmed pan lined with buttered parchment, sized so the batter sits about an inch deep as the recipe specifies; a half-sheet is too large for this quantity at that depth. Without a rim the batter will run.
- Apricot marmaladeApricot jam, warmed and pushed through a sieve, brushed on thin. It seals the crumb so the water icing (confectioners' sugar with orange juice, thick enough to coat a spoon) does not soak in.
- YieldCut into small squares, a sheet of this size gives a couple of dozen pieces, depending on the pan you use. Trim the baked surface as the recipe directs, and cut after the icing has set.
Cakes built on eggs, sugar and ground almonds with little or no wheat run through Iberian confectionery, and tradition links much of it to convent kitchens where egg yolks were said to be plentiful. The name "Portuguese" appears to have been used loosely in nineteenth-century professional cookery for sponges of this type flavored with orange. Refined potato starch was widely available as a European pastry ingredient by the nineteenth century and was valued for the fine, dry crumb it gave sponge cakes that had to be trimmed, layered and iced, which is exactly the use here. A thin sheet sponge masked with apricot glaze and finished with a flavored water icing was standard work in a professional pastry section, cut into small squares for tea trays and banquet service.