I used regular boiled and peeled chestnuts. The shape doesn't matter, so you can use ones that have been split in half and scooped out with a spoon.
I used about 300 g of peeled chestnuts. The amount of sugar to aim for is about 30% of the chestnuts in weight, and the amount of water is about 20%.
Combine sugar and water in a saucepan and heat to make syrup. When the sugar has melted, turn the heat down, add the chestnuts, and mix while simmering for a little bit.
While there is still moisture left in the pan, add mirin and salt and bring back to a simmer. Turn off the heat.
Puree in a food processor or pass through a strainer and it's done.
If you are passing it through a strainer, it's easier to do so if you don't simmer the chestnuts for too long in Step 4. (If needed, pass it through a strainer first and then simmer the paste a little more.) A superb chestnut paste is done.
To make the Mont Blanc topping: I passed the chestnut paste through a fine meshed sieve again, and added 1 coffee creamer (5 ml) to thin it out. I also added a little vanilla essence.
Story Behind this Recipe
The rule in our house is to eat the first chestnuts of the season simply by boiling them. I turned the leftover chestnuts into paste. It's so fun to think what I'm going to make with the paste.
There is little syrup, so be careful not to burn it on the bottom in Step 3. If you have a dried gardenia fruit to add yellow color, add one at the syrup making stage. If you puree the paste in a food processor, a few little lumps will remain. If that bothers you or you plan to use it for something like Mont Blanc, I recommend passing the paste through a strainer.