175g unsalted butter, softened
175g caster sugar
3 large eggs
1 tsp vanilla extract
175g cassava, sifted
25g cocoa powder
1 tsp baking powder
1 tsp gum xathan
2 tbsp milk
50g dark chocolate 70% cocoa solids
Preheat the oven to 170c/150f/gas mark 3.
Butter and line the base of a 900g/1 lb loaf tin with greaseproof paper.
Beat together the butter and sugar until light and fluffy, then beat in the eggs one at a time. Add the vanilla. Fold in the cassava, baking powder and gum xathan. Add the milk to loosen the batter.
Divide the batter between two bowls. Stir the cocoa powder into the mixture in one of the bowls. Using a tablespoon dollop the chocolate and vanilla mixes into the tin alternately. When all the mixture is used up, tap the base of the tin to level the batter. Take a skewer and swirl it around the batter in the tin a few times to create the marble design.
Bake for 45-55mis until a skewer inserted in the middle comes out clean.
Leave to cool in the tin for 15 mins then transfer to a cooling rack . Melt the chocolate in a heatproof bowl set over a pan of simmering water. Drizzle the melted chocolate over the top of the cake and leave to cool. This cake will keep for 3 days in an airtight container.
If the cake is browing too much put a layer of tin foil over the surface of the cake during cooking