Preheat the oven to 180ºC. Grease the inside of a 20cm deep round cake tin, or a 27 x 14 loaf tin and line the base with baking paper.
Gently mix the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition. Don’t overdo it.
Add the mashed banana and vanilla, mix to combine .
Stir the baking soda into the warmed (not hot) milk and add to creamed mixture.
Sift the dry ingredients together and fold through the creamed mixture. Be sure to measure this quantity precisely.
Scoop the mixture into the prepared tin, level, and bake for one hour or until cake springs back when lightly touched. This timing depends on the even heat provided by your oven, so some ovens might take 50 minutes. Keep an eye on it!
Leave the cake in the tin for 15 minutes before placing on a wire rack to cool.