Sift the self-raising flour (150g), coffee powder (10g) into a large bowl and salt (⅛ tsp). Add the remaining ingredients (3 medium eggs, 165g soft margarine, 165g soft light brown sugar and 1½ tsp milk) and mix until fully combined (either by hand or using an electric mixer on a low speed). Fold in the chopped walnuts (75g).
Divide the mixture equally between the three tins.
Bake for 20-25 minutes until a skewer inserted into the middle comes out clean. Leave the cakes to cool in their tins for about 10 minutes, then remove them from their tins and move them to a wire rack to cool fully before decorating.
Make the coffee buttercream
Mix the instant coffee granules (3 tbsp) with the boiling water (1½ tbsp) and then set aside to cool.
Cut the butter (250g) into cubes and then beat on a low-medium speed until soft. Gradually add the icing sugar (you add it gradually to stop it flying everywhere) and beat until fully combined.
Add the vanilla extract (¼tsp) and add the coffee a teaspoon at a time, beating in each addition before adding more.
Check the consistency of your buttercream. It should be soft and perfect for piping at this point. If it’s still a little stiff then beat in a little milk.
Place the bottom layer of your cake onto your chosen dish. Cover with ⅓ of your buttercream. Sprinkle over half of the chopped walnuts.
Place another layer on top and repeat the last step.
Place the final layer on top. Decorate with the remaining buttercream and then finish with the walnut halves.
I've decorated my cakes by piping buttercream roses and then filling any gaps around the edge with a little extra squirt of buttercream. I used a JEM 1B nozzle.
If you'd prefer to make this as a 2-layer cake then the only change you'll need is to increase the cooking time to 30-35 minutes. You'll also need ⅔ of the buttercream (or you could use the extra buttercream to make a thin coating around the outside of the cake.
If you'd like to bake the cake in one deep tin and then slice it up into layers, you'll need to increase the cooking time to 45-50 minutes.
The undecorated cakes are suitable for freezing. Simply wrap them tightly in clingfilm and pop them into the freezer. Defrost them thoroughly before decorating.
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