Peel the banana, cut into thin slices and mix with the lemon juice. Mix the flour with the baking powder, the cornstarch and the lemon zest. Mix the egg with the oil, the sugar, buttermilk and sour cream and sift in the flour mixture.
Fold well until all ingredients are moist. Preheat the oven to 180°C (approximately 350°F). Line the muffin cups with paper cases and fill halfway with the batter.
Place the banana slices over top and cover with the remaining batter. Bake in preheated oven until golden brown, about 20 minutes. Loosen from the wells, remove and leave to cool on a kitchen rack. Serve with coffee or tea.