Combine the flour, cocoa powder, sugar, and salt in the bowl of a food processor and process until well combined, about 30 seconds. Cut the butter into small pieces. add it to the processor and process until the butter is quite crumbly and broken up to about the size of peas.

Add the creme fraiche, in two or three installments, pulsing after each addition, until it is all incorporated. The dough should stick together when pinched between your fingers.

Dump the dough onto a sheet of plastic wrap and shape it into a disc. Wrap tightly in the plastic and place in the refrigerator for about an hour. A bit less is fine. Preheat the oven to 350 degrees and lightly spray a 24 cup mini muffin pan with cooking spray. set aside.

In a large bowl, combine the eggs, yolk, and vanilla and lightly whisk. Add the salt and sugar and whisk again, until fully incorporated. Add the melted butter and whisk again. And finally add the pecans and whisk (or fold) them into the filling. Set aside. I left mine on the counter, but you can refrigerate it, if you'd prefer.

Remove the dough from the refrigerator and form balls, about the size of 1 tbsp, plus 1 tsp. Press the balls into the bottom of each mini cup in your pan. Using your fingers, evenly press the dough up the sides of the cup, til it just extends a bit past the top. The dough should be equally thick along the sides and bottom of the cups. Repeat until all the cups are lined with dough.

Whisk your filling to re-incorporate it, and place a tablespoon of filling in each cup of your pan. Place about 6 or so mini chocolate chips, if using, on top of the filling.

Place the pan in the oven for 20 minutes, rotating the pan after 10. Let the pan cool on a wire rack until the tassies are cool enough to handle and use your fingers and a paring knife to remove the tassies from the pan. Transfer them to the wire rack to continue cooling. Tassies will keep Tightly covered on the counter for 3 days.