Why Was My Coffee And Walnut Layer Cake Icing So Runny?

Our answer

Nigella's Coffee And Walnut Layer Cake (from KITCHEN) is the classic British favourite but bolstered with extra coffee flavour. Ground walnuts and espresso powder are added to the sponge and the cake is filled and topped with a coffee buttercream.

The buttercream is a simple mixture of butter and icing (confectioners') sugar and the ratio of double the amount of icing sugar to butter is a fairly standard amount. The amount of liquid added to the buttercream is very small, so we doubt that this is the problem. Runny buttercream is usually caused by one of two things. The first is temperature. If the kitchen is very warm and humid or if the butter itself is too warm and soft then the icing will be runny. The butter needs to be soft enough so that if you press it with your finger then you leave an indent, but not so soft that you can squish the butter with your finger. If the butter is too soft then it will soften further (or even melt slightly) as it is beaten and will give a runny buttercream. If you put the buttercream in the fridge then it should start to firm up and become spreadable rather then runny. Refrigerate for a couple of hours and then check, if the buttercream is still too warm then continue to chill it and check every 30 minutes until the buttercream is firm enough to spread. Once you have frosted the cake, make sure that you store it in a cool place. Also, it is possible to beat the buttercream for too long, particularly with a stand mixer, and in this case the buttercream can form too many air pockets. When these deflate the buttercream becomes runny. Unfortunately, if this happens once the buttercream is on the cake then it is difficult to fix, but try to process or beat the buttercream just until it is smooth and uniform and not for any longer.