Preheat oven to 400°. Bring a medium sized pot of water to a boil and turn off heat.
Soak the chestnuts in the hot water for 1 minute then drain chestnuts and pat dry. (This softens the shells and makes them easier to score.)
Using a sharp paring knife, carefully score each chestnut - cut an X on the rounded side (I have also read flat, but I do rounded) of each chestnut while holding them down firmly on a kitchen towel.
Arrange the scored chestnuts in a single layer, X side up on a sheet pan or cast iron pan.
Roast until the peel begins to curl up, about 20-25 minutes.
A few minutes before the chestnuts are done, make the salted coffee butter. Heat the butter and maple syrup in a small saucepot until bubbly. Remove from the heat and add the instant coffee and vanilla, stirring until the coffee is dissolved.
Remove chestnuts from the oven and place them X side up in a serving dish or platter. Pour the coffee butter over the nuts making sure to get the sauce into the cracks of the chestnuts. Place a clean towel or napkin over the chestnuts and left sit for 2 to 3 minutes until the chestnuts are cool enough to handle. Season generously with flaky salt and serve!