Line a 12-section muffin tray with paper muffin cases. Lightly brush a foil-lined baking sheet with oil and scatter with the rhubarb. Sprinkle with 25 g (1 oz) of the muscovado sugar and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–30 minutes or until tender and beginning to darken around the edges. Leave to cool. Reduce the oven to 180°C (350°F), Gas Mark 4.
Put a further 150 g (5 oz) of the sugar, 150 g (5 oz) of the butter, 175 g (6 oz) of the flour, the baking powder, cinnamon and eggs in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Divide the cake mixture between the paper cases, spreading it fairly level, and top with the rhubarb pieces.
Put the remaining butter and flour in a food processor and process until the mixture resembles coarse breadcrumbs. Add the remaining muscovado sugar and process briefly until mixed. Scatter over the cakes and sprinkle with the almonds.
Bake for 25–30 minutes, until risen and golden. Transfer to a wire rack to cool. Serve dusted with icing sugar.