Preheat oven to 375 degrees and line a 9x9-inch pan with parchment (bottom and 2 sides only, leaving an overhang to easily remove the bars).
In the bowl of a mixer, combine butter, sugar and vanilla; mix 1 to 2 minutes.
Add salt, almond flour and coconut flour, beating until fully combined and a dough has formed.
Separate pieces of dough over the bottom of prepared pan, and press evenly to cover. Bake until lightly browned, 12 to 13 minutes. Cool completely on a rack.
Once the base is cool, make the truffle topping by heating the cream to just about boiling (over medium heat in a saucepan or in the microwave for 1 to 2 minutes). Remove from heat and add the chocolate pieces and salt. Whisk until smooth.
Pour the truffle topping over the cooled base and spread to cover evenly. Refrigerate until completely set, 2 to 4 hours (or overnight).
Remove from pan using parchment ends and cut into 36 small bars. Refrigerate (or freeze) in a single layer until ready to serve.
Notes
Storage:
Refrigerate: These bars are best kept in the refrigerator and brought out a few minutes before serving. They are good about a week.
Freeze: They can also be frozen - add them to an airtight container for longest storage (2-4 months) or simply in a pan or cookie sheet covered in plastic and foil if you need them in 2 to 4 weeks.