Put the egg yolk and sugar in a bowl. (If possible, warm the bowl over a water bath.) Whip until white and thick.
Heat the milk in the microwave until it's just about to boil. Add to the bowl from Step 2 and mix well.
Rinse the kabocha squash. Wrap in plastic wrap without patting it dry, and microwave for 3 minutes to soften. Peel, and weigh out 100 g.
Put the egg mixture from Step 3, the kabocha squash, and the cinnamon powder into a blender. Blend until smooth, and transfer to a bowl.
Microwave the gelatin for 15 seconds to dissolve. Add to the bowl from Step 5 and mix well. Chill the bowl in a bowl of ice cold water or in the fridge.
Remove the bowl from the ice cold water (or take it out of the fridge) when the gelatin begins to harden and the mixture becomes thick. It should be as thick as a potage soup.
In a separate bowl, whip the heavy cream until thick and creamy. Whip until slightly more airy than the mixture from Step 7.
Add the whipped cream to the bowl from Step 7, and mix until everything is evenly blended. When mixed, pour into a container and chill in the fridge.
For best results, chill for minimum 3 hours. Scoop with a large spoon to serve.
Story Behind this Recipe
This mousse is perfect for dessert, since it's light and chilled.
Adjust the amount of milk to the consistency of your kabocha squash. The thickness of the mousse should be similar to that of a thick potage soup. If you over whip the cream, the mousse won't have a good texture. Stop whipping once the cream has thickened.