Preheat the oven to 180°C/160°C fan/350°F/gas mark 4. Grease a 23cm / 9 inch square cake tin and line it with baking parchment.
Place the flour, cocoa powder, baking powder, bicarbonate of soda and salt in a large bowl and whisk to combine.
In a separate bowl, mash the bananas very well. Add the sugar, oil, milk, vinegar and vanilla extract and whisk until well combined.
Pour the wet ingredients into the dry and stir with a balloon whisk until no dry lumps remain. Add the chocolate chips and fold in gently.
Pour the batter into the prepared tin, spread it level and bake for 25-30 minutes until the cake is firm to the touch and a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin for 20 minutes then carefully turn it out onto a wire rack and leave it to cool completely.
To make the ganache, place the finely chopped chocolate in a heat-proof bowl and set aside.
Place the coconut milk, peanut butter and a pinch of salt in a saucepan. Stir over a low heat until melted and just coming up to a simmer; do not let it boil.
Pour the hot mixture over the chopped chocolate and leave it to stand for 5 minutes. Do not stir it until the five minutes is up.
Stir the ganache gently until melted and smooth.
Leave the ganache to cool and firm up, this can take several hours. You can pop it in the fridge to speed it up but check it frequently and give it an occasional stir. It can go from not-quite-set to solid quite quickly so keep a eye on it.
If the ganache does become too firm; place the bowl over a pan of gently simmering water (don’t let the base of the bowl touch the water). Stir for a minute or so until it softens up. Remove the bowl from the heat before it has fully softened to the right consistency as the residual heat from the bowl will continue to melt the ganache.
Spread the ganache over the top of the cooled cake and serve.