Soak the linseeds in the water for about 5 minutes until the water has been absorbed and it has gone a little jelly like.
Meanwhile, cream the coconut oil and sugar until well combined, then mash in the banana.
Stir well, then beat in the soaked linseeds.
Sieve in the dry ingredients, then add the nectar and vinegar. Stir until just combined.
Spoon into a 23 cm round silicone cake mould or tin and bake at 180℃ (350℉, Gas 4) for about 35 minutes when the cake should be risen and a skewer inserted into the middle comes out clean. Turn onto a rack to cool.
Melt the remaining coconut oil with the chocolate and nectar in a pan over very low heat.
Stir until just combined.
Remove from the heat, allow to cool slightly, then pour over the cooled cake.