Preheat the oven to 180C/350F/gas 4. Grease 2 x 20cm sandwich tins and line each base with baking paper. Dissolve 2 heaped teaspoons of coffee in one tablespoon of boiling water. Set aside 20 pecan halves and finely chop the rest.
Put 225g/8oz of the butter and the caster sugar in a large bowl and beat until light and creamy. Add the eggs, one at a time, beating well after each addition and adding a tablespoon of the flour with the last two.
Fold in the rest of the flour, the dissolved coffee and the chopped nuts. Divide between the cake tins and level the tops. Bake for about 25 minutes or until the top springs back when lightly pressed. Turn out and cool on a wire rack.
Dissolve the last teaspoon of coffee in a tablespoon of boiling water and leave to cool. Beat the remaining butter and icing sugar together until light and creamy. Add the cold coffee.
Sandwich the cakes together with half the buttercream and spread the rest on top. Decorate with pecans and serve.