Preheat the oven to 160 °C.
Mix the flour, sugar, baking powder and bicarbonate in a bowl. In another bowl, mix butter, eggs, milk, vanilla extract and mashed bananas. Add to the flour mixture and mix well.
Pour the batter into the greased pan and bake in the oven for 45-50 minutes. Prick with the tip of a knife to test for doneness: it must be clean when pulled out.
Leave to cool for 5 minutes and then remove from the mold.