Preheat your oven to 180 degrees Celsius (355 Fahrenheit degrees) and line an 8-inch square tin with baking paper;
In a bowl mash the bananas until smooth with a fork (or alternatively in a blender);
In a separate bowl combine together the buckwheat flour, sugar, vanilla, baking powder and apple cider vinegar.
Then combine the plant milk and maple syrup to the mashed bananas. Pour it into the dry ingredients and combine well. Then pour the melted coconut oil and stir with a whisk removing any lumps. Do not over stir.
Pour the cake batter in your cake tin and top with 2 sliced bananas. Cook at 180 degrees Celsius (355 Fahrenheit degrees) for 50-60 minutes or until a skewer inserted comes out clean. Leave to cool on a wire rack.