Grapefruit Sandwich Cake

Name: Grapefruit Sandwich Cake
Ingredients: 80g/3 oz unsalted butter; 80g/3 oz sugar; 3 large free-range eggs; 125ml/4fl oz grapefruit juice (around 2½ grapefruits); 1 grapefruit, zest only; 1 grapefruit; 75g/2½ oz caster sugar, plus extra for coating; 200g/7 oz caster sugar; 200g/7 oz baking spread; 210g/7½ oz self-raising flour; 4 large free-range eggs; 25g/1 oz ground almonds; 1 grapefruit, zest grated and half the juice; 75ml/2½fl oz grapefruit juice (around 1½ grapefruits); 75g/2½ oz caster sugar; 100g/3½ oz mascarpone; 200g/7 oz double cream; 50g/1¾ oz icing sugar, sifted