With the rack in the middle position, preheat the oven to 190 °C (375 °F). Line 12 muffin cups with paper liners.
In a bowl, coarsely mash the banana with the lemon juice. Set aside.
In another bowl, combine the flour and baking powder. Set aside.
In a third bowl, cream the butter, brown sugar, and peanut butter with an electric mixer. Add the eggs, one at a time, and beat until smooth.
At low speed, add the dry ingredients alternately with the mashed banana, milk, and vanilla.
Spoon the batter into the prepared cups. Sprinkle with the peanuts. Bake for about 30 minutes or until a toothpick inserted in the centre of a muffin comes out clean. Remove from the cups and let cool on a wire rack.