Preheat oven to 190ºC.
Prepare a baking sheet by dusting with flour.
Stir the lemon juice into the milk then put aside.
Sieve the flour, baking powder and bicarbonate of soda into a large bowl.
Mix in the salt, chopped white chocolate, and mashed banana.
Add the milk and lemon mixture and mix to a dough with a fork (add a little more flour if necessary).
Bring together to form a rough ball of dough (you can still use the fork).
Tip onto the baking tray and flatten with your hand. (Speed is required to get this in the oven asap as the raising agents start working as soon as he liquid is added).
Dust the top with flour (using a sieve), then cut a deep cross into the dough.
Bake on middle shelf of oven for 30 minutes or until the base sounds hollow when knocked.
Cool on a wire rack.
Serve warm or cold with butter.
Will keep for a couple of days.