Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Grease a box form (28 cm long; 1.7 l capacity) and sprinkle with semolina. Peel sweet potato, grate coarsely.
Peel 1 banana and mash it with a fork. Mix with 100 ml oil, buttermilk, eggs and 2 tablespoons honey. Mix flour, sugar, baking powder, 1 pinch of salt and vanilla. Add banana mixture and mix briefly. Fold in the sweet potato rasp.
Peel 3 bananas and halve them lengthwise. Place them close together on the bottom of the mould. Drizzle 2 tablespoons of honey over the bananas. Spread the dough on top and smooth it down. Bake in a hot oven for about 70 minutes (test with sticks!). Take out, let rest in the pan for about 5 minutes. Turn out onto a plate. Let it cool down. Serve sprinkled with some honey.