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Chocolate Filling
1 1/2 squares chocolate 1/4 cup milk 1 cup brown sugar 1 egg-yolk 1/2 teaspoon butter 1 teaspoon vanilla Melt the chocolate over hot water, in a double boiler; add the milk, and cook together, stirring until the mixture is thick and creamy. Add sugar and beaten egg-yolk, stir until smooth and cook five minutes. Add the butter. Beat well. Remove from heat and add flavoring. Cool before spreading on cake.
A cooked chocolate filling of the old school, thickened with an egg yolk rather than confectioners' sugar. It comes together in a few minutes and depends almost entirely on keeping the heat gentle.
- Squares of chocolateA square means one ounce of unsweetened baking chocolate, so this calls for 1 1/2 oz. Do not substitute semisweet or chocolate chips without cutting the brown sugar sharply.
- The hinge: keep it over waterThe yolk is what sets this filling, and it curdles at a lower temperature than the mixture will reach over direct heat. Keep the double boiler water at a bare simmer and never let it touch the upper pan.
- Brown sugarMeasure it packed. Light brown gives a cleaner chocolate flavour; dark brown pushes it toward a fudge-and-molasses taste.
- Temper the yolkThe instruction to add the beaten yolk straight to the hot chocolate works only if you do it slowly with steady stirring. Easier is to stir a spoonful or two of the hot mixture into the yolk first, then return the lot to the pan.
- Five minutesThat is enough to cook the yolk through and thicken the filling to a heavy ribbon. Five minutes over hot water should also bring the mixture up near the temperature at which egg yolk is considered cooked, though a double boiler kept at a bare simmer will not necessarily reach it; use a thermometer if you want to be sure.
- Vanilla off the heatAdding it after the pan leaves the stove is deliberate. Vanilla is volatile and much of the aroma boils away if it goes in earlier.
- Cooling and consistencyIt thickens considerably as it cools and will tear a cake crumb if spread while stiff. Cool to just warm, then spread; if it sets too hard, beat in a teaspoon of milk.
- YieldRoughly three quarters of a cup to a cup, enough between the layers of one 8- or 9-inch two-layer cake. Double it if you also want to cover the top.
Boiled and cooked fillings like this one belonged to a period when a cake was assembled in layers and the filling between them was often treated as a distinct preparation from the frosting on top, and cookbooks of the 1920s and 1930s frequently give fillings their own heading. The New Delineator Recipes was published under the name of a women's magazine, and its cake chapter assumes a home baker with unsweetened baking chocolate, a double boiler and brown sugar on hand. The egg yolk here does the work that cornstarch or a quantity of powdered sugar would do in a later recipe, giving a filling closer in texture to a soft custard than to the buttercreams that followed.