Let's make the sponge cake! Add sugar to the eggs, and mix together just like making meringue (This might take a bit of time.)
Add 2 to 1, and mix some more. Add lemon peel and vanilla extract after the mixture is no longer powdery, then briskly mix.
Spread parchment paper in a pan, and spread out the batter evenly to the edges of the pan. Bake for 12 minutes at 360F/180C in an oven.
Microwave or boil all of the kabocha to cook through, mash and strain the kabocha once it has softened.
Let's making the cream filling! Thoroughly whip sugar and heavy cream together. Mix butter into kabocha measured out from Step 5, and whip together.
Let's make the topping cream! Whip the remaining kabocha, heavy cream, and sugar together, and place it into a Mont Blanc frosting bag.
Use the edge of a knife to cut shallow lines into the cooled dough and spread the filling cream out on top. (Spread it thickly at the front, and make a bit of a mound at the end.)
After wrapping, put the overlap at the bottom, and let it sit in the fridge for an hour.
Squeeze lots of kabocha squash cream from Step 7 over the roll cake to finish!
Story Behind this Recipe
I simplified a recipe I learned in college.
In Step 4, you should pour the dough from 30 cm above to eliminate air bubbles. If you do not have a Mont Blanc pastry bag, put the whipped cream in a thick plastic bag, cut 1 cm out of the corner, and squeeze on top. You can also add rum if you wish, and it'll be delicious .