Peel and seed kabocha, and cut into chunks. Place in a microwave-safe bowl and cover with plastic wrap. Microwave for about 4 minutes, or until pumpkin is fully cooked. Smash down through a strainer and let it cool.
Whip heavy cream to stiff peaks. Fold in kabocha puree. Transfer to a piping bag and set aside.
Heat a pot over medium heat; add butter, garlic and ginger. Sauté until you can smell aroma, about 2 minutes. Add chicken and cook until edges brown, about 3 to 4 minutes. Add carrot, onion, mushrooms and curry powder. Cook for 1 minute. Add tomatoes, stock, sugar and soy. Break down tomatoes with a wooden stirring spoon. Bring it to boil and stir occasionally. Reduce heat to low and let simmer for 5 minutes.
Mix cornstarch with 2 teaspoons cold water and pour into curry while stirring. You will see curry is thick and shiny; remove from heat and set aside.
Cook udon by following directions on package. Drain completely and toss in curry.
Place curry udon into a serving bowl and completely cover top with pumpkin mousse. Sprinkle black sesame seeds on top and enjoy!