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Chocolate Filling
2 1/2 squares unsweetened chocolate 1 cup powdered sugar 3 tablespoons milk Yolk 1 egg 1/2 teaspoon Vanilla Melt chocolate over hot water, add one-half the sugar, and milk; add remaining sugar, and yolk of egg; then cook in double boiler until it thickens, stirring constantly at first, that mixture may be perfectly smooth. Cool slightly, flavor, and spread.
A cooked chocolate custard filling, thick enough to hold between cake layers without soaking them. It goes wrong in only one place: overheated egg yolk.
- Chocolate squaresA square of unsweetened chocolate is one ounce, so this is 2 1/2 oz. Baking bars are now usually scored into 1 oz sections; if yours is not, weigh it.
- "Powdered sugar"In American books of this period powdered sugar often meant finely ground granulated sugar, not the modern cornstarch-cut confectioners' sugar. Either works here since the mixture is cooked, though the cornstarch in confectioners' sugar may set it slightly firmer.
- The hingeOnce the yolk is in, keep the water in the lower pan below a simmer. A sugared, milk-slackened yolk mixture thickens somewhere around 175-180F and will curdle if pushed much past that, and no amount of stirring will bring a curdled mixture back smooth.
- Judging thicknessCook until it coats the back of a spoon and holds a line drawn through it with a finger, roughly 5 to 8 minutes over gently steaming water. It thickens further as it cools.
- Flavoring lastVanilla is added off the heat for a reason: the alcohol carries the aroma off if it goes in hot. Cool to about lukewarm first.
- Spreading temperatureWarm filling runs off the layers; cold filling tears the crumb. Spread when it is just cool to the touch but still spreadable.
- YieldRoughly a cupful, enough for one layer of an 8- or 9-inch cake. Double it for a two-layer filling.
- KeepingIt contains egg and milk, so refrigerate any leftover and use the filled cake within two days. Bring back to spreading consistency over warm water, not direct heat.