In a bowl, combine all the ingredients except for the chocolate and mix with a whisk. Break up the chocolate and add half to the batter, folding in with a spatula.
Lightly grease the steam case with oil and pour in the batter. Top with the remaining broken chocolate pieces.
Put on the lid and cook in the microwave at 600 W for 2:30~2:40 minutes. Let rest for 3 minutes with the lid on.
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Story Behind this Recipe
I made this while keeping in mind that a batter without egg would be firm so the chocolate would not sink.
You can use bar chocolate or baking chocolate. Any kind of chocolate, such as dark or milk, is fine. Please adjust the baking time according to the size of your steam case (It will also vary by the room temperature.)