Make the coating: Combine the cacao nibs, Demerara sugar and salt in a food processor and pulse until the mixture is the consistency of very coarse sand. Transfer to a small bowl and set aside.

Make the dough: Add the flour, powdered sugar, cocoa powder and salt to the food processor and pulse to combine. Add the butter pieces and process until the butter breaks down into very fine bits. In a small bowl, whisk together the egg, sour cream and vanilla until smooth, then sprinkle over the flour mixture. Process until a smooth dough forms.

Chill the dough: Scrape the dough onto a clean work surface, cut into 3 equal pieces, roll each into a log, flatten into rectangles, wrap in plastic, and chill for at least 1 hour.

Make the filling: Melt chocolate and butter over simmering water, then mix in powdered sugar, flour, egg yolk, and vanilla until smooth. Let cool.

Roll out the dough and fill: Roll out a piece of dough, spread with melted chocolate mixture, and roll into a log. Repeat with remaining dough.

Heat the oven and bake: Preheat oven to 350 degrees. Line a baking sheet with parchment paper.

Coat the cookies: Brush logs with egg white, roll in cacao nib mixture, and chill until firm.

Slice and bake the cookies: Cut logs into segments, arrange on baking sheet, and bake until puffed and firm. Cool completely and dust with powdered sugar before serving.