Preheat oven to 180°C/355°F (you may add a tray of water in the oven to increase the humidity, which will make the cake moister).
Mix and sieve together flour, baking powder, baking soda and salt.
Blend banana and milk together until smooth.
Combine margarine and sugar in a mixing bowl, then beat over medium speed until light and fluffy.
Add in beaten eggs gradually and beat until fully combined.
Sieve in ⅓ part of flour, mix until combined using rubber spatula.
Add in ⅓ part of banana puree, mix until combined.
Repeat the same steps until all flour and banana puree have been added, then continue folding until the batter is well mixed.
Tap mixing bowl on countertop for 3-4 times, then pour batter slowly into 2 greased and lined loaf pans (8″ x 4″, each about 540 g of batter).
Drop the loaf pans on table for 4-5 times (to burst large air bubbles), then use a wet knife to draw a line horizontally across the batter (to help the cakes cracking in the center).
Bake at 180°C/355°F for 45-50 minutes, or until fully cooked (The cake is done when the wooden stick inserted in the center of the cake comes out clean). (Reminder: The heat for different oven is different, the suggested time is only for reference, adjust the baking time base on your oven if necessary.)
Let the cakes cool in the pans for 10 minutes before unmoulding.
After unmoulding, let the cakes cool on wire rack for 30-35 minutes before serving.