Preheat the oven at 350F/180C for 15 minutes. Lightly grease a 9×13 inch pan and line it with parchment paper.
In a large bowl sift together the flour, baking powder, baking soda and salt. Mix in the sugar.
Make a well in the center of the dry ingredients and stir in the the wet ingredients one by one. Stir the mixture together using a whisk. Some lumps is okay.
Pour the batter in the prepared pan and tap the pan to even it out and break the air bubbles if any.
Bake it for 25-30 minutes. Transfer the pan to a cooling rack and remove the cake after 15-20 minutes. Place the cake on the rack for it to cool down completely before slicing it.Cut cooled cake in half horizontally.
For the icing, put butter, icing sugar, vanilla and coffee into a large bowl. Beat with a handheld electric whisk until soft and smooth. Use some of the icing to sandwich cake together, then spread remainder over top and sides of cake. Transfer to a cake stand or plate.
Set aside 8 walnut halves, and finely chop the rest. Press chopped walnuts around sides of cake and top with walnut halves. Serve in slices.
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