Preheat oven to gas mark 4 / 177*C / 350F.
In a large bowl add together: 250g of caster sugar, 250g of flour, 2 eggs, half a teaspoon of baking soda, 1 can of crushed pineapple and 30g of crushed walnuts.
Using Bakewell baking paper line a square baking tin.
Add the mixture to the tin and place in the middle of the oven.
Bake for 40 minutes.
When time is up remove the cake and let cool on a wire rack.
Whilst the cake is cooling melt 50g of dark chocolate in a glass bowl over boiling water in a saucepan, adding 10g of melted butter, stirring continuously
Add the chocolate on top of the cake, adding additional whole walnuts. Leave to cool and serve.