Cake Filling

Three ingredients and no numbers beyond the cup measure, so everything depends on reading the cooking stage correctly. This is a boiled sugar filling in the penuche family, not a soft frosting.

• The hinge: the saucer test"Dries when beaten in saucer" means you drop a spoonful onto a cold plate, stir it with the spoon, and watch for it to lose its gloss and set firm rather than spreading. That behaviour is roughly what a modern cook would recognise around the soft-ball stage, though the recipe gives no temperature; pulled off much earlier the filling will run out from between the layers.
• Sour cream then and nowIn this period sour cream generally meant cream that had soured naturally, typically richer than today's cultured sour cream. Modern full-fat cultured sour cream works, but a mix of heavy cream with a spoonful of sour cream gives a closer result and is less prone to breaking.
• Curdling is expectedThe cream will look separated and grainy in the first few minutes as the acid and heat hit the proteins. It comes back together as the sugar concentrates. Do not try to rescue it with more cream.
• Pan and heatUse a heavy saucepan two or three times the volume of the mixture and keep the heat genuinely low; milk solids scorch fast at sugar-syrup temperatures and a burnt note carries through the whole filling. Stir throughout, reaching the corners.
• NutsThe recipe says only "nuts"; walnuts, hickory nuts or pecans, chopped medium, all suit. It does not say when they go in. Stirring them in off the heat once the mixture has reached stage keeps them crisper than cooking them in the hot sugar.
• SpreadingBeat briefly off the heat until it thickens and clouds, then spread while still warm. It sets hard as it cools, so work quickly; if it stiffens too far, a teaspoon of hot cream will loosen it.
• YieldRoughly a cup and a half of filling, enough for the layers of one 8- or 9-inch two-layer cake, or for a thin coating over the top as well.