Combine and whisk egg yolk, milk and sugar in a bowl. In a separate bowl, whip egg whites to stiff peaks. Combine ⅓ of whites into the yolk mixture, whisking to loosen the batter.
Sift in cake flour, matcha powder, baking powder and salt; whisk into the wet mixture till combined. Fold in the remaining whites. Avoid over-whisking at the flour addition stage.
Move slider to COMBI-STEAM mode and select STEAM BAKE function on the SmartLid™ to preheat. Add water just below the basket attachment. Line 6 ramekins with cupcake liners and distribute the batter.
Add 10g adzuki beans to each ramekin. Once preheating is complete, place ramekins on the basket attachment. Close the lid and STEAM BAKE for 12 minutes.
Once done, check the mushipan with a toothpick; it should be dry. Unmold the mushipan and let it cool before serving.
