For the pineapple sheet cake, knead flour, cocoa, sugar, mineral water, oil, eggs, vanilla pulp and baking powder into a smooth dough.
Drain the pineapple pieces in a sieve. Line a baking tray with baking paper, place the dough on it and spread the pineapple pieces evenly on it.
Bake at 180 °C for at least half an hour (test with chopsticks).