Chocolate-Hazelnut Rugelach

Pulse together flour, granulated sugar, and salt in a food processor until combined, 3 to 4 pulses. Add cream cheese and butter; pulse until mixture forms large, crumbly pieces, 6 to 8 pulses. Add egg yolk; pulse until just incorporated, 3 to 4 pulses.  

Turn dough out onto a clean work surface, and knead until dough just comes together, about 1 minute. Divide in half; shape each piece into a rectangle about 5 x 3 inches. Wrap individually in plastic wrap, and chill at least 2 hours or up to overnight (8 hours). 

Stir together brown sugar and ground cinnamon in a small bowl. Remove 1 chilled dough rectangle from refrigerator, and unwrap. Place dough on a well-floured work surface, and lightly dust with flour. Roll into a rectangle about 12 x 6 inches. Sprinkle dough rectangle with half of the brown sugar mixture. Sprinkle half of the chopped hazelnuts over brown sugar mixture. Sprinkle half of the finely chopped chocolate over hazelnuts. Starting at 1 long end, carefully roll dough up into a tight log. Place log, seam side down, on a baking sheet. Repeat procedure with remaining chilled dough rectangle, brown sugar mixture, chopped hazelnuts, and chopped chocolate. Freeze logs, unwrapped, on baking sheet 15 minutes.

Preheat oven to 375°F. Remove 1 dough log from freezer; cut into 12 (1-inch-thick) slices. Arrange slices, standing upright with seam side down, on a baking sheet lined with parchment paper. Brush tops of slices evenly with half of the whipping cream, and sprinkle with half of the sparkling sugar. Bake until rugelach are golden brown and dough is flaky, 13 to 15 minutes. Cool on baking sheet 5 minutes; transfer to a wire rack, and cool to room temperature, about 20 minutes. Repeat procedure with remaining dough log, whipping cream, and sparkling sugar.