Preparation: Put butter in a bowl and bring to room temperature. Sift the ◎ ingredients together. Bring the egg(s) to room temperature too.
Whisk butter until smooth. Add sugar in 2 to 3 batches, mixing well between additions.
Beat the egg well and add to the butter little by little Preheat the oven to 350°F/180°C.
Add the ◎ dry ingredients in 2 to 3 batches, cutting in with a rubber spatula.
Divide the batter evenly in lined muffin cups, and smooth out the surface.
If you are adding toppings, put them on now. Bake the cupcakes in the preheated oven for about 15 minutes.
If a skewer inserted in the middle comes out without raw batter on it, the cupcakes are done. Cover loosely with plastic wrap while the cupcakes are still warm, let cool and serve.
If adding chestnuts: Brush the tops with the syrup the chestnuts are packed in. When I use chestnuts, I reduce the sugar in the batter a little, but that's up to you.
Cherry blossom version: I tried using salt preserved cherry blossoms in the spring, and they were delicious - I recommend them. De-salt the cherry blossoms first, and top on the cupcakes in Step 6.
To add anko (not listed): Make small balls of anko using plastic wrap, and partially freeze. Drop into the the batter and bake. I received feedback from users and it looked delicious, so I tried it myself.
Wrap to give as gifts. I think the cupcakes taste better and are moist after a day. You can freeze them too.
Bonus: You can add green tea liqueur too. Add liqueur after Step 1. Mix in the ◎ ingredients first, to prevent the batter from separating. You can also sprinkle a little liqueur on the cakes after baking.
Bonus: I topped the cupcakes with adzuki bean flavored round chocolates from Hokkaido that my friend sent me. I used mini-cupcakes (35 ml), and baked for 11 minutes or so.
Bonus: I topped the cupcakes with my favorite "Kinako chocolate black beans" from Ugetsu. After letting the cupcakes cool for a day, the black bean becomes soft and is a nice accent. I recommend this if you can get these chocolates.