Peel sweet potatoes, grate coarsely. Finely chop pineapple and ginger with a large chef's knife. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease a box form (approx. 25 cm long; approx. 1 1⁄2 l capacity) with oil.
Mix grated sweet potatoes, pineapple, ginger, 200 ml oil, 400 g soy yoghurt and 100 g syrup with the whisk of the mixer. Mix flour, baking powder and baking soda. Stir the flour mixture into the sweet potato mixture.
Put the dough into the mould and smooth it down. Bake in a hot oven for approx. 1 hour (chopstick test), covering with baking paper after 45 minutes if necessary.
Take the cake out of the oven. Leave to cool in the mould for about 45 minutes. Carefully drop onto a cake rack and let it cool down. Just before serving, pour the remaining yoghurt onto the cake. Drizzle 1 tablespoon of syrup over the cake.