Ahhh this is tricky, kesian mummy! So toddler dia suka chocolate tapi constipation lah pulak masalahnya. One of my mum friends punya anak sama je, suka Dutch Lady chocolate; she tried mixing the vanilla slowly into the chocolate one (like 80% chocolate, 20% vanilla) and eventually boleh adjust anak dia, sikit-sikit lah. But if sembelit jadi issue, better check with the paediatrician la first sebab mungkin anak tu perlu formula lain or lactose situation, or maybe kena add more fibre/water juga. In the meantime, maybe try other brown susu brands ke, or ask the doctor if the chocolate formula itself yang causing it atau boleh tetap cocoa tapi kena tweak nutrition side lain, sebab gain weight pun important but comfort tu lebih dulu kan.