If I wanted to bake the Coffee And Walnut Layer Cake as cupcakes rather than the standard two cakes, how long would the mixture need in the oven? I would use a regular sized muffin tin.
Sponge layer cakes, such as Nigella's Coffee And Walnut Layer Cake (from KITCHEN) convert fairly well to cupcakes. A 20cm/8-inch two layer cake is likely to make 20-24 cupcakes. You should fill the paper cupcake or muffin cases half full with cake batter. The oven temperature will be the same but the baking time is likely to be 15-20 minutes (depending on the size of the cupcake cases). The cakes should be risen and firm to the touch and a cake tester inserted into one of the cakes in the centre of the pan should come out clean.
The quantity of buttercream icing in the recipe will give a modest layer on top of each cupcake, but if you want to pipe big swirls then you are likely to need 1.5x to 2x the quantity stated in the recipe.