Hard Sauce

A three-ingredient sauce that lives or dies on temperature: the butter must be soft enough to cream but never warm enough to turn oily, or the sauce separates into greasy sugar.

• Powdered sugarIn American books of this era "powdered sugar" often meant granulated sugar ground fine, closer to today's superfine than to confectioners'. Modern confectioners' sugar works and gives a smoother result.
• Butter temperatureHave the butter at cool room temperature, around 65F. Cream it until pale and light before any sugar goes in, then add the sugar in three or four additions so it stays emulsified.
• The odd fractionsThe thirds and two-thirds suggest a larger formula scaled down. Read them as generous pinches: about 1/4 teaspoon lemon extract and 3/4 teaspoon vanilla will not go wrong.
• Measurements1/3 cup butter is a little over 5 tablespoons, or about 75 g. One cup of confectioners' sugar is roughly 120 g.
• ChillingHard sauce takes its name from being served firm. Mound or pipe it into a dish, or press it into a shallow pan and cut in rounds, then chill at least an hour before serving.
• Yield and servingMakes about 3/4 cup, enough for four to six portions of steamed or plum pudding. Serve the pudding hot so the sauce melts against it; that contrast is the whole point.
• If it curdlesAn oily or grainy sauce means the butter got too warm. Refrigerate briefly, then beat again until it comes back together.