Bring the butter and egg to room temperature. Sift the ☆ marked ingredients together.
Peel the skin and discard the seeds from the kabocha (110 g with out seeds and skin) and cut into small pieces. Microwave until soft.
Place the kabocha into a bowl and roughly mash with a pestle or something similar. Mix in 20 g of sugar and let it cool.
In another bowl, beat the butter until creamy. Add 40 g of sugar and beat until it becomes whitish.
Add the beaten egg to the butter little by little.(See the Helpful Hints) Mix in the kabocha and rum.
Fold in the sifted ☆ marked ingredients with a rubber spatula.
Pour the batter into the mold. Drop it on the table a few times to get rid of the air bubbles. Smooth the surface with a rubber spatula and let it sit for 20 minutes. (This will allow it to rise easily.)
Bake it in the 180℃ preheated oven for 10 minutes. Wet a knife with water and score the cake by making a shallow lengthwise cut. Bake for another 30 minutes.
Stick a toothpick through the center, and if it comes out clean, it's done. Remove from the mold and let it cool on a rack.
Story Behind this Recipe
I made a pound cake with seasonal kabocha squash.
If the egg is cold, or if it is added all at once, the batter will separate. Make sure you bring the egg to room temperature and add it in small batches, mixing well each time. If the batter separates, add about 1 tablespoon of and combine.