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Old-Fashioned Chocolate Filling
3 squares melted chocolate 3 tablespoons cream 1 egg 3/4 cup powdered sugar 1 tablespoon corn starch 1/8 teaspoon salt 1 teaspoon vanilla extract Add the melted chocolate and the cream to the well-beaten egg; mix in the powdered sugar gradually; add corn starch which has been mixed until smooth in a little cold water; cook in top of double boiler, stirring constantly until smooth and thick; add salt and vanilla. Spread between layers of cake.
A cooked chocolate custard filling, thickened with both egg and cornstarch. It comes together in ten minutes once the water is hot, and the only way to ruin it is to let it get too hot too fast.
- Squares of chocolateA "square" is one ounce of unsweetened baking chocolate, so this calls for 3 oz. Chocolate sold in bars scored into squares today is usually sweetened; if you substitute semisweet, cut the powdered sugar back by about a third.
- Powdered sugarIn American books of this period "powdered sugar" often meant a fine granulated sugar rather than modern confectioners' sugar, which is milled finer and already contains cornstarch. Either works here since the filling is cooked, but confectioners' sugar will make it slightly thicker """ a good reason not to overshoot on the cornstarch.
- CreamThree tablespoons is a small amount, so its richness counts; heavy cream will give a fuller result than half-and-half or milk, though the recipe specifies only "cream."
- The hinge: gentle heatThe water in the lower pan should be hot but not at a rolling boil. Overheated egg can curdle into grainy specks that stirring will not smooth out, though the sugar and cornstarch here raise the temperature at which the egg sets. Stir constantly and pull it off the heat as soon as it coats the spoon heavily.
- Slurry the cornstarch"A little cold water" means a tablespoon or two """ just enough to make a lump-free paste. Added dry, the cornstarch will seize into knots the moment it meets the warm mixture.
- Vanilla lastAdding the vanilla off the heat, as directed, is deliberate: the flavor is volatile and simmering it away wastes it.
- YieldRoughly a cup, enough to fill a two-layer 8- or 9-inch cake. It is a filling, not a frosting -- it sets soft and will not hold up on the sides of a cake.
- StorageEgg-thickened and unbaked, so keep the finished cake refrigerated and eat it within two or three days.
The New Royal Cook Book was one of a series of promotional cookbooks issued by the Royal Baking Powder Company, which distributed branded recipe booklets widely to encourage regular use of a single product. A filling like this one belongs to the layer-cake era -- quick batters baked in shallow tins, then stacked with something soft between them. The cornstarch is the insurance policy -- it lets a single egg thicken the mixture without the cook having to bring it to the point where the egg alone would set, which is part of the reason the double boiler is specified.