Preheat your oven to 180℃. Either boil the potato or heat in the microwave, then mash. While still hot, add the butter and granulated sugar and mix. Once it's generally cooled, set aside 1 teaspoon of the egg and mix the rest in well.
Combine and sift the cake flour, joshin flour, and baking powder, and add to Step 1. Mix lightly. Use a spoon to scoop a spoonful onto a baking pan lined in a cookie sheet.
Use a brush or your fingers to apply the leftover egg as a coating. Bake in the oven at 180℃ for 15-18 minutes.
Once they're finished baking, allow to cool on a cake rack.
Story Behind this Recipe
I wanted to make simple yet delicious desserts made from great-tasting and full-of-nutrients potatoes that come from the northern region of Japan.
In Step 3 you can adjust the shape while applying the egg; so in Step 2, as long as you have the basic size the shape doesn't have to be pretty. For this reason, I recommend using your fingers in Step 3 rather than a brush.