The Frosting Dam

A frosting dam is a thick ring of stiff buttercream or cream cheese frosting piped around the outer edge of each cake layer before any wet filling - jam, curd, compote or ganache - goes in. It does two jobs at once. Mechanically, the dam holds the filling inside the cake, preventing it from squeezing out of the sides under the weight of the next layer. And moisture-wise, the butter-rich frosting is fat-based and hydrophobic, so it resists the capillary migration of sugar-water from the filling into the cut face of the sponge - the failure mode that softens the crumb and destabilises the build. Pipe the dam from a bag with a snipped tip or a plain round nozzle, slightly taller than the filling will sit, and chill the part-built cake for fifteen minutes between layers for extra security before adding the next sponge.