In the bowl of a food processor or using a pastry cutter, cut the butter into the flour, pulsing until you have large crumbs. Add the egg yolk and creme fraiche and continue to pulse (or knead by hand) until the dough comes together in a ball. Divide the dough into three equal parts and form into balls. I like to use a food scale to equally divide the dough, but you can also eyeball it. Wrap each portion of dough in plastic wrap, flatten into a disk, and chill in the refrigerator for 2-3 hours or overnight.
Make the filling by tossing together the toffee bits, sugar, and cinnamon in a small bowl. If the toffee bits have large chunks, I like to pulse it a few times in a food processor so the filling is a more even crumb.
Preheat the oven to 350° F with a rack placed in the center of the oven. Prepare a baking sheet and line it with parchment paper.
On a floured surface, roll out 1 ball of dough into a 10-inch circle about 1/8 inch thick. Sprinkle with the filling, pressing it down into the dough, then cut into 12 equal wedges. Starting at the outside edge, roll each wedge from the widest to the smallest point, creating a crescent shape. Work as quickly as you can - it's easier to work with the dough when it is still nice and cool. Place the cookies on a parchment lined baking tray. Chill in the refrigerator for about 10 minutes.
Repeat with the other two pieces of dough.
Right before baking, lightly brush with the egg white, and if you wish, sprinkle with sanding sugar. Bake for about 18-20 minutes, or until the cookies are golden. Transfer to a wire rack to cool completely.