Caramel Filling for Cake

This is a boiled brown-sugar fondant of the simplest kind — three ingredients and a timed boil. The whole result depends on where the syrup lands on the sugar scale, so a thermometer does more for it than the clock does.

• Weights into cupsA pound of brown sugar is about 2 1/4 cups, firmly packed. A "lump of butter size of a small egg" is roughly 2 ounces, or 4 tablespoons.
• Cream or milkHalf a cup of cream, or a little less milk; presumably the smaller measure of milk allows for the extra water it carries. Modern heavy cream works directly; whole milk may need a little longer on the boil.
• The five minutes is a proxyBoil to the soft-ball stage, 236-240F, rather than trusting the timer. Five minutes at a hard rolling boil in a wide pan reaches that point; five minutes at a simmer in a narrow one will not, and the filling stays syrup.
• Pan choiceUse a heavy saucepan holding at least two quarts. Brown sugar and dairy foam up considerably and scorch easily on thin metal.
• Beat before spreadingThe mixture goes on the cake as a thick, opaque, spreadable mass, not as hot syrup. Cool it to about 110F, then beat with a spoon or mixer until it loses its gloss and thickens; add the vanilla at this stage so the flavor is not driven off by the heat.
• If it sets too hardBeat in a spoonful of hot cream or milk at a time until it spreads. Overcooked sugar candy filling is easily loosened; undercooked filling cannot be rescued except by reboiling.
• FlavoringA teaspoon of vanilla suits this quantity, though the original leaves it to the cook. A pinch of salt, which the original omits, keeps the sweetness from flattening out.
• YieldEnough to fill and thinly top two 8- or 9-inch layers. Work quickly once it thickens, as it firms further as it cools.