Line a standard 2lb loaf tin with greaseproof paper. Preheat you oven to 180°C/160°C fan.
Cream together your butter and sugar for 3-5 minutes.
Add your eggs one at a time, fully incorporating after each addition. Be sure to scrape down the sides of the bowl in between.
Add the soured cream and vanilla and mix until combined. Then add your dry ingredients except the demerara sugar and mix again until just combined.
If your bananas are actually ripe, then there's no need to mash them. Just throw them into the mixture in chunks and beat until broken into pieces and evenly distributed. If you don't like some chunks of banana though, then by all means mash thoroughly before adding.
Pour the batter into the prepared tin and level out the surface. Cut an extra peeled banana in half lengthways. Place the banana halves, cut-side up, on top of the batter and sprinkle with the demerara sugar.
Bake 55-65 minutes, or until a skewer inserted into the centre of the banana bread comes out clean. Remove from the tin and leave cool.
The banana bread will last well at room temperature for three days.