Whip the eggs in a double boiler until soft peaks form. Preheat the oven to 320F/160C.
Add the kuromitsu and matcha to Step 1 and mix well. Add the ingredients, and mix in well with a rubber spatula. (Add 5 g or so of chopped sweet beans or other additions if you prefer.)
Pour the batter into milk carton baking molds (seeor a poundcake tin lined with parchment paper. Drop the mold or tin lightly on a countertop to even out the batter. (Sprinkle the remaining sweet beans or other additions on top.)
Bake for 35 minutes at 320F/160C and it's done. If you are using a milk carton mold, cut the 4 corners with scissors, run a knife along the bottom and remove the cake.
Wrap in plastic wrap to prevent the surface from drying out.
Variation: For a chocolate version, use cocoa powder instead of matcha and dried fruits instead of sweet beans.