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Foamy Sauce
½ cup butter
2 tablespoons hot water
1 cup confectioners' sugar
1 teaspoon vanilla
1 egg
Cream the butter and gradually add the sugar, the egg, well beaten, and the hot water. Heat over hot water, beating continually until it thickens. Add the vanilla and serve.
A hot foamy sauce is a butter sabayon: the egg does the thickening, and the whole thing lives or dies by how gently it is heated. Everything else is assembly.
- The hinge: gentle heat, constant beatingSet the bowl over barely simmering water, not touching it, and never let the mixture stall while you fetch something. If the egg sets faster than you can beat it in, the sauce breaks into buttery curds and will not come back.
- How thick is thickYou are looking for a pourable foam that coats a spoon and holds a brief ribbon, which takes several minutes of steady whisking. By the time it thickens it should be at or near 160F, the temperature at which egg is generally reckoned safely cooked, so there is no reason to stop short.
- Measurements in modern termsHalf a cup of butter is 4 oz or one stick. A cup of confectioners' sugar is about 4 oz by weight; today's version carries a little cornstarch, which does no harm here.
- Creaming the butterButter at cool room temperature, around 65F, creams properly with the sugar. Straight from the refrigerator it will stay in lumps that show up later as grease slicks on the surface of the finished sauce.
- The hot waterTwo tablespoons of hot water loosen the creamed base so the egg incorporates smoothly. Add it last, after the egg, as written, and add it in a thin stream rather than all at once.
- Vanilla off the heatVanilla goes in after the sauce leaves the pan. Its aromatics are volatile and simmering them off is the difference between a perfumed sauce and a faintly sweet one.
- Yield and serviceAbout one and a quarter cups, enough for six to eight servings over steamed pudding, gingerbread or baked apples. It is served warm and will slacken and separate if held; make it while the pudding is unmoulding.
- If it breaksPull the bowl off the heat immediately and whisk in a tablespoon of cold water or a spoonful of cold butter. Caught in the first few seconds it will often pull back together; past that, start again.
Foamy sauce belongs to the family of hot egg-and-butter sauces that American cookbooks ranged under "pudding sauces" -- a category that has now nearly vanished with the steamed puddings it was made for. The technique is a domesticated sabayon, the Italian zabaglione and French sabayon both being egg yolks beaten with sugar over hot water, though the American version swaps wine for butter and vanilla and uses the whole egg. Butterick's book appeared when the pattern company was extending its authority from dressmaking into the whole apparatus of the middle-class household, and a section of sauces like this one is a good index of what that household ate: plain boiled and steamed puddings, cottage puddings, gingerbread, all of them dry and all of them redeemed by something rich poured over the top.