Preheat the oven to 190°C.
Butter and line two 20cm sandwich tins with baking parchment.
Pulse walnuts in a food processor until chopped into small pieces.
Blend butter, sugar, eggs, flour, and baking powder until well combined.
Fold the chopped walnuts into the batter.
Divide the mixture into tins and bake for 25 minutes.
Cool cakes in tins for five minutes before transferring to a wire rack.
Blend icing butter, icing sugar, and 150g of fudge until smooth.
Sandwich the cakes together with half of the icing.
Spread remaining icing on top and decorate with walnut halves and remaining fudge.