The cake I baked always 'explodes' in the center. Some said is the way of beating (after adding the eggs because eggs trap air inside the cake), some said is the heat distribution of the oven.
Tonight I tested again with a Marble cake mix which I bought some time ago.
I used the middle rack in the oven. First I turned on the top and bottom heat mode. After 20 minutes or so, when the cake slightly expanded in the center, I turned the heat mode to bottom only.
And it turns out great! Should experiment with other cakes :)
By the way, the instruction states that divide the batter into half and mix the cocoa powder into 1 halve. I shall make it 1/3 cocoa and 2/3 butter the next time I bake Marble cake, too much cocoa makes the cake too bitter.