Preheat the oven to Gas 4, 180°C, fan 160°C.
Butter a 1kg (2lb) loaf tin or use 12 large muffins cases.
Cream the butter and sugar until smooth and then whisk in the mashed bananas. Add the eggs, vanilla, cinnamon and a pinch of salt and whisk well. Sieve the flour and baking powder and beat until smooth. Mix the bicarbonate of soda into the milk and stir into the batter. Toss the walnuts in a little flour before adding to the mixture. This will prevent them from sinking.
Scrape the mixture into the tin and bake for about 50 minutes, until the bread is crusty on the top and a skewer poked into the middle comes out clean. Cool in the tin, and then turn out onto a cooling rack. Cook the smaller muffins for 20-25 minutes.
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