Add the lemon juice to the coconut milk and let stand to curdle while you get on with the next steps
Preheat the oven to gas 3 / 160C / 325F and grease and line a 24" round cake tin
Cream together the margarine and sugar until pale and fluffy then beat in the vanilla and lemon zest
Sift together the flour, xanthan gum, baking powder and cinnamon, mix well then fold into the creamed margarine / sugar
Stir in the milk then gently mix in the chopped rhubarb
Pour the batter into the lined cake tin. The batter will be thick but the rhubarb will release juices while it bakes
Mix the topping ingredients together and sprinkle evenly over the batter
Bake for 75 minutes or until a skewer inserted in the middle comes out clean with no raw batter on it. If the top of the cake begins to burn before it's done cover it loosely with foil
Let stand in the tin for 5 minutes before releasing and letting cool on a wire rack before slicing
Delicious served warm with some hot custard
