Line a 12-hole nonstick muffin tin with paper muffin cases or lightly grease. Sift the plain flour, baking powder, bicarbonate of soda and salt into a large bowl, then stir in the oats and fudge chunks.
Beat together the remaining ingredients in a jug or bowl. Pour into the dry ingredients and stir until just combined.
Spoon the mixture into the prepared muffin tin and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 18–22 minutes until risen, golden and firm to the touch. Serve warm.