Preheat oven to 180°C
Line a loaf pan with baking paper
Using an electric mixer, cream butter and sugar, add beaten eggs slowly, mixing as you pour
Stir in banana
Sift flour and baking powder on top
Add milk. Stir with a wooden spoon until combined
Fold in the mixed berries
Spoon mixture into loaf pan
Bake for 45 to 50 minutes