Marshmallow Frosting

A boiled-sugar frosting that uses commercial marshmallows in place of the beaten egg white of a seven-minute icing. The whole thing sets as it cools, so the final line is the important one.

• Work fast at the endMarshmallow frosting stiffens as it cools and quickly becomes unspreadable. Have the cake cooled, level and within reach before you start beating the syrup in.
• The milk syrupTwo tablespoons of milk with six of sugar is a very small volume in a saucepan. Use the smallest pan you own, keep the heat moderate, and watch it: at this scale six minutes on a modern high-output burner may carry it further than intended. Judge by consistency and colour rather than the clock.
• MarshmallowsA quarter pound is 4 oz, roughly 16 large marshmallows or about 2 cups of miniatures. Today's marshmallows are dusted with a starch coating, and they may want a splash more than the two tablespoons of boiling water to come smooth.
• Double boilerKeep the water at a bare simmer and the bowl clear of it. Marshmallow scorches readily over direct heat and turns grainy and brown-tasting.
• "Beat in hot sugar"The syrup goes into the melted marshmallow, not the reverse, and it should be poured in a thin stream while beating so it incorporates instead of setting into a lump. An electric hand mixer does in two minutes what a spoon does in ten.
• VanillaA third of a teaspoon is an odd measure; a scant half teaspoon is close enough, added off the heat so it does not flash off.
• YieldA modest amount of frosting, enough for the top of a single layer. Double it to fill and cover a two-layer cake.
• StorageIt crusts on the surface within a few hours and does not keep or re-warm well. Frost the day you serve.