Preheat the oven to 180°C. Grease a small loaf tin (450 - 500 g / 1 lb) with a little butter and line the base with baking paper.
Put the sugar and butter in a mixing bowl and beat with electric beaters for a couple of minutes. Lightly whisk the egg and vanilla with a fork. Mash the bananas with the yogurt. Add both to the bowl and beat well.
Sift in the flour, baking powder and cinnamon and fold in until just combined. Roughly chop the pecans and fold in. Pour into the prepared tin.
Bake for 45 minutes, or until a skewer inserted in centre comes out clean. Leave in the tin on a wire rack for at least 10 minutes before serving warm, or turn out onto the rack to cool completely.
Double the quantities for a standard (1 kg / 2 lb) loaf tin and bake for an hour to an hour and 10 minutes (test with a skewer after an hour).