Carefully score chestnuts with an "x" just piercing the shell and place on baking sheet. It'll take some force to get through the outer shell. Use a sharp knife and keep fingers out of the way!
Roast for 30 minutes until chestnuts have split open and peeled back a bit.
Remove from oven and let cool for a couple of minutes until you can handle them. Remove shell from chestnuts and place flesh in a food processor. Make sure to peel them while they're still warm/hot as it's much easier than when they cool.
Add walnuts and pecans to the food processor and process for about 2-3 minutes until finely chopped.
With the food processor running, add the coconut oil. Let it continue processing for about 2-3 more minutes then add honey, vanilla and salt.
Scrape down if needed then continue processing and add water until the mixture starts to come together a bit more.
Stop the food processor when the mixture can be pinched together with your finger and is essentially "spreadable". It will not be as creamy as a regular nut butter.