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.
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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.