Cut the pumpkin into quarters and remove the seeds.
Put the pumpkin pieces in a microwave-safe bowl. Add ½ cup of water, cover with a lid or wrap, and poke a few holes. Microwave for 5–7 minutes, or until tender when pierced with chopsticks.
Once cooked, peel off the pumpkin skin.
Add sugar and salt to the pumpkin, then mash well with a fork or masher until smooth.
Add glutinous rice flour to the mashed pumpkin and mix until it forms a dough.
Line a steamer with a cloth or parchment paper. Poke small holes in the paper so steam can pass through, and lightly oil the surface.
Steam the dough over medium heat for 10 minutes with the lid closed. It’s ready when the color turns slightly translucent and no white flour is visible inside.
Remove the steamed dough and knead it while it’s hot to make it chewy. Flatten it into a thick, wide sheet. (Tip: Use a spatula first, then wear plastic gloves lightly brushed with sesame oil. The dough is sticky when hot.)
Blend or finely grind the white part of the Castella cake (or sponge cake) in a chopper. You can also sift it through a fine sieve for a finer texture.
Sprinkle the finely ground cake evenly over the surface of the rice cake.