Remove the seeds and peel from the kabocha, cut into chunks, wrap in plastic, and microwave. Smash into a paste while still hot, let cool, and mix in the sugar.
Let the butter and egg come to room temperature. Preheat the oven to 350F/180C.
Knead the butter until soft, then mix in the white sugar.
Whisk the egg.
Combine flour, cocoa powder, and baking powder and sift into the egg, mixing well.
Divide Steps 1 and 5 into 15 portions each, then wrap Step 5 around Step 1. Lay on a baking paper-lined cookie sheet.
Decorate each with 1 kabocha seed. Bake for 15 minutes at 350F/180C.
Once baked, transfer them on top of the parchment paper to a flat serving dish. If not serving immediately, cover with cling wrap to prevent drying.
At my house, when making manju, we spread out the dough on a piece of cling wrap, place the filling in the center, and use the cling wrap to form the manju. Of course, do whatever works best for you.
Story Behind this Recipe
I wanted a recipe I could make with my kids.
When adding sugar to the kabocha in Step 1, if the kabocha is still hot, it could get too mushy, so be sure to chill before mixing. When microwaving the kabocha, 4:30~5 minutes on 500~600W should do the trick. Adjust cooking times based on your microwave and oven.