Toss the rhubarb with the 3 tablespoons sugar. Tip on to a baking sheet lined with baking parchment. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes until softened. Leave to cool.
Put the remaining sugar in a bowl with the flour and baking powder. Stir in the marzipan and cooled rhubarb. Beat together the eggs, melted butter and milk in a jug. Add to the dry ingredients and stir together using a large metal spoon until just combined.
Divide the mixture evenly between paper muffin cases arranged in a 12-hole muffin tray. Bake for about 20 minutes until well risen and just firm to the touch. Transfer to a wire rack to cool, then serve warm or cold, sprinkled with sifted icing sugar.