I baked the cakes and I noticed the surface of the top layer is not flat. I am quite lost now, should I trim the top to make it level? or just leave it as it is. Cos the instruction said to bake and cool and fill.

I looked on Amazon for some customer comments on these pans and it sounds like you will have to level your layers carefully to make sure your "tunnel" meets in all the right places. One commenter emphasized that the batter must rise at least to the top of the pan to make sure the tunnel works right.

My bottom layer did rise to the top of the pan but the top did not. Maybe I should stick to the recipes included ;(