Why did my eggless cup cake prepared with condensed milk get charred in a convection microwave?
It's probably best to make this cake in an actual oven (even if the recipe specified a microwave oven) as the heat will be more even. Anyway, ovens can be remarkably inaccurate in temperature so perhaps although you set your oven at 200 C it may have been actually at 225 C say. The batter may have also been quite thick and so have a higher tendency to burn unlike a thinner, wetter batter, if you pair this with the inaccuracy of the oven temperature then it's not surprising that it burnt.

Hope this helps for next time!