Preheat the oven to 190°C, gas mark 5. Line a 12-hole muffin tin with 10 muffin cases. Place the rhubarb and brown sugar in a small pan with 1 tbsp of water and cook gently for about five minutes until tender. Continue to cook very gently. Sift the flour, baking powder, bicarbonate of soda and ginger into a mixing bowl. Stir in the caster sugar.
Whisk together the eggs, soured cream and butter. Make a well in the centre of the dry ingredients and add the whisked ingredients. Fold in the fudge chunks and cooked rhubarb to combine, then spoon into the muffin cases.
Bake for 20-25 minutes until golden and firm to the touch. Serve warm with vanilla ice cream or transfer to a wire rack to cool.