There are a few reasons why your buns came out like this. - You could have over-proofed the dough. Over-proofing the dough weakens the dough structure and so when you steam it may collapse. - Air bubbles when shaping the dough. When shaping the dough make sure you squeeze out all the air bubbles before you rest the dough for 15 minutes. If you leave the air bubbles in the dough they will yield weird indents in the bun. - Steaming too hot. When you steam the buns over high heat, the steam makes the buns grow too quickly and so when you take them out of the steamer they immediately shrink and become wrinkly.