Orange Filling

A flour-thickened citrus curd of the kind Fannie Farmer's kitchen favored over pure egg-and-butter curds — cheaper, more forgiving, and stiff enough to hold between cake layers without soaking them.

• The hinge: cook the flour outTen minutes over simmering water is not padding. Flour needs sustained heat to lose its raw, pasty taste and to swell fully; pull it at five minutes and the filling tastes chalky and slackens as it sits.
• Mix in order givenNote that the printed order puts the orange juice before the flour. Adding juice to loose flour gives lumps that no amount of stirring in the double boiler will undo, so it helps to combine the sugar and flour before any liquid goes in.
• Double boilerWater simmering, not boiling, and the bowl clear of it. Direct heat will scramble the egg into threads. If you use a heavy saucepan over low heat instead, stir into the corners constantly and expect to strain the result.
• "Grated rind 1½ orange"Zest — only the colored layer, none of the white pith. Grate it before juicing, and zest into the sugar so the oils are rubbed into the crystals.
• The eggOne egg, slightly beaten, stirred in cold with everything else. Ten minutes over hot water carries it well past the point of setting, so no separate tempering step is needed.
• ButterA teaspoon is all. The recipe mixes it in with everything else before cooking; stirring it in at the end off the heat instead gives a little gloss. Either way it is a finishing touch, not a thickener.
• YieldRoughly three-quarters to one cup — enough to fill one layer of an 8- or 9-inch cake. Double it for a three-layer cake.
• CoolingPress plastic wrap or buttered paper directly on the surface or it will skin over. It thickens noticeably as it cools; spread it cold, not warm, or it will run out from between the layers.