1 cup self-raising flour
2 tablespoons plain flour
¼ cup Dutch-pressed cocoa powder
¾ cup caster sugar
100g dark cooking chocolate, chopped
125g unsalted butter, melted
2/3 cup sour cream
2 teaspoons vanilla extract
¾ cup soft, over-ripe mashed banana
Line a 6-hole Texan muffin pan (3/4-cup capacity) with large paper muffin wraps.
Sift combined flours and cocoa into a large bowl. Stir in sugar and half the chocolate.
Place butter, egg, sour cream and vanilla in a large jug. Whisk well. Add to flour mixture with banana. Stir with a wooden spoon until just combined.
Spoon evenly into muffin wraps. Sprinkle with remaining chocolate.
Cook in a moderate oven (180C) for about 25 minutes, or until muffins spring back when lightly touched in the centre. Remove from oven. Stand in pan 5 minutes.
Transfer muffins to a wire rack to cool before serving.