This is the recipe for the sweet chestnut paste I used for this custard: (https://cookpad.com/en/recipes/170074-kuri-an-sweet-chestnut-paste)
Combine the soft brown sugar and cornstarch in a bowl, and mix well with a whisk. Add 20 ml of the milk and mix well.
Add the egg into the batter from Step 2, and mix well. Add the sweet chestnut paste and mix.
Add the rest of the milk, a little at a time, into the batter from Step 3 and mix well until even.
Microwave uncovered for 1 minute and 30 seconds at 600 W. Remove from the microwave and mix with a whisk. Repeat once more. Mix thoroughly.
Check the texture of the cream, and microwave to your desired texture, if necessary. Add the finely chopped sweet chestnuts in syrup and the essence, and mix.
Tightly wrap the cream with plastic wrap without air pockets and thoroughly chill. Then it's done.
It's delicious to use as a filling for cream bread rolls, or for seasonal tarts.