Gelatin is the key to a glistening chocolate glaze that stays put on your cake. Jacques Torres prepared this recipe with Martha on Episode 501 of "Martha Bakes."
In a saucepan, combine 1/2 cup water, sugar, heavy cream and cocoa powder. Simmer for about 20 minutes until mixture slightly thickens.
Place 1 tablespoon cold water in a small bowl. Sprinkle gelatin over top and let soften 5 minutes. Remove chocolate mixture from the heat and whisk in gelatin mixture until thoroughly combined.
Set over an ice bath and whisk until the glaze thickens and reaches about 80 degrees, about 1 minute.
This recipe makes enough glaze for one 8" two-layer cake. Pour over chilled crumb-coated cake placed on a wire rack set over a large bowl. Use an offset spatula to smooth out the glaze. Work quickly as the glaze will set up in a minute or less.