Grapefruit Sandwich Cake

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