Heat the oven to 200C (180C fan). Line a 1kg loaf tin baking paper, making sure some hangs over the edges, (to use as handle to remove the loaf later), and grease the sides with a little oil.
Put the buckwheat flour, hemp protein, hemp hearts, baking powder, bicarb and salt in a large bowl and mix to distribute everything evenly.
In another bowl, use a fork to mash 400g bananas to a paste, then add the tahini, sugar, oil, vinegar, vanilla, and milk. Mix well, then scrape into the flour bowl and mix until the batter is smooth and uniform.
Pour into the lined tin, then tap it on a work bench to level out the surface and release any air bubbles. Thinly slice the remaining banana and lay these on top of the cake to decorate and sprinkle with extra hemp hearts.
Bake for 30 minutes, then turn down the oven to 180C (160C fan), turn the tin around and bake for a further 30 minutes, until the top of the bread is golden, and a skewer comes out clean.
Take out of the oven, leave to cool in the tin, then lift out using the baking paper “handles”. Cut into thick slices, spread with vegan butter, and serve.
