Preheat oven to 350F. Line two baking sheets with parchment paper.
Using the bowl of a stand mixer fitted with the paddle attachment or using a hand-held mixer, cream the butter and sugar until light and fluffy. Add the egg and crème fraiche and mix until smooth. Scrape your mixer well and beat for another few seconds. Add the flours, cocoa powder, baking soda, salt and cocoa nibs, if using.
Scoop about 1 small tablespoon of dough and place 1” apart on your baking sheets. Bake for about 8 minutes. Cool completely before sandwiching with chocolate filling.
CHOCOLATE FILLING
Melt butter in a small pan or in the microwave.Remove from heat and stir in chocolate until it has melted. Let this rest to spreadable consistency.
Spread a small amount on one cookie and using another cookie, sandwich the filling.