Why Did My Chocolate Sheet Cake Icing Not Set?

Our answer

Nigella's Chocolate Sheet Cake is similar to the popular American chocolate sheet cakes (sometimes called a Texas sheet cake). The cake is baked in a rectangular pan and is slightly thinner than many cakes. The icing is poured over the cake while it is still warm. The warm cake absorbs some of the icing, giving the cake a fudgy consistency with a glaze that cracks slightly when you bite into it.

It is really important that the cake is still warm when the icing is poured over, so that the cake can absorb some of the frosting. If the cake is cold then the crust will have set fully, which means the icing can't sink in and will sit on the surface and be runnier. And the cake will then also not have the same fudgy texture. Occasionally the icing doesn't set well and this is usually in warmer weather, so you could try refrigerating the cake for a couple of hours. When you make the icing it should have a thick but pourable consistency and if it is too thin then it may also not set. If you are using a spreadable type of butter it may not set as well as this type of butter has extra water and vegetable oils in it, which makes it soft to spread but means the icing will not set. Also, measure the milk carefully - it is 3 tablespoons, which is 45mls. If the icing looks too thin (like a glacé icing) then you can add a little extra icing (confectioner's) sugar until you get a thicker consistency.