Preheat the oven to 180ºC, gas mark 4. Sift 200g of the flour with the baking powder, bicarbonate of soda and the cinnamon into a large mixing bowl. Rub in 75g of the butter until the mixture looks like coarse breadcrumbs and stir in 100g of the sugar. Beat the eggs with the yogurt then beat into the dry ingredients to make a soft mixture.
Spoon the cake mixture into a buttered and base-lined baking tin, about 28x18x4cm, and smooth the surface. Top with the rhubarb. Rub the remaining butter and flour together and stir in the rest of the sugar. Add the chopped macadamia nuts and the white chocolate chunks. Scatter over the rhubarb.
Bake for 35-40 minutes until well risen and a skewer inserted into the centre of the cake comes out clean. Turn onto a wire rack to cool. Serve warm in slices with Waitrose Seriously Creamy Madagascan Vanilla Ice Cream or leave to cool and store in an airtight container.
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