Break or chop up the white chocolate into a microwave-safe container. Add butter and microwave for 1 minute at 600 W to melt.
Mix with a whisk.
Add the egg, pancake mix, sugar, and matcha. Mix well.
Grease a microwave-safe mini gateau mold (or small cupcake liners) with vegetable oil. Pour in the cake batter with a spoon.
The cake rises significantly when baking, so don't overfill with the batter.
Drop the mold onto a table a few times to remove any air pockets.
Microwave each mold in two batches. Keep your eyes on them, and microwave for 2 minutes at 600 W. Let it rest in the microwave for 2 minutes. Repeat twice for each batch.
The microwave will be warm when heating the mold a second time, so shorten the heating time slightly.
When it's slightly cooled, remove the cake from the mold and let it cool. Use a tea strainer to sprinkle matcha or powdered sugar on top.
It became moist the next day.
Here's a gateau with milk chocolate. (Recipe ID: 2070199)
"Gateau au Chocolat in a Silicone Steamer" Recipe ID: 1251677
Story Behind this Recipe
I saw something similar in a dessert booklet and modified "Gateau au Chocolat in a Silicone Steamer" (Recipe ID: 1251677) for a matcha version.
Adjust the microwave time to the mold and the amount of batter. You'll end up with holes in the cake when there is air trapped in the dough. In that case, sprinkle with matcha or powdered sugar on top.