Preheat the oven to 180°C/160°C Fan/Gas Mark 4. Grease and line a 900g loaf tin. Place the butter and sugar in a bowl and cream together until smooth.
Add the eggs, Vanilla Extract, mashed banana and yogurt, beat the mixture until all combined.
Sieve the flour and Baking Powder on top and fold into the mixture until just combined. Pour the mixture into the prepared loaf tin.
Slice the remaining banana in half length ways and sprinkle with sugar. Place the banana cut side up on top of the mixture in the loaf tin, ensure the bananas are towards the edge of the loaf to prevent them from sinking. Place in the oven and bake for about an hour until a skewer inserted into the centre of the cake comes out clean. If the cake starts to brown too much on top, cover with foil and continue to bake. Leave to cool in the tin for 20 minutes and then place on a wire rack to cool completely.
Your banana bread will keep for about 5 days in an airtight container.