1) Preheat oven to 180C/Gas 4. Grease a 12-cup capacity muffin tin or use paper cases.
2) With a wooden spoon, mix butter into the mashed bananas in a large mixing bowl. Mix in the sugar, egg, coffee and vanilla. Sprinkle the bicarbonate of soda and salt over the mixture and mix in. Add the flour, mix until it is just incorporated. Fold in the chopped pecans. Pour mixture into a prepared muffin tin.
3) Bake for 20-30 minutes. To check if the muffins are done, insert a toothpick into the centre of a muffin. If it comes out clean, it's done.
Cool muffins on a rack and serve.