Make the dough: Process butter, cream cheese, and powdered sugar in a food processor fitted with the blade attachment until smooth, about 1 minute. Add zest, vanilla, and egg yolk; pulse until combined, about 4 pulses. Add flour and salt; pulse until dough comes together in a soft ball, about 8 pulses.

Place dough on a sheet of plastic wrap, and pat into a 1/2-inch-thick rectangle; cover with the plastic wrap, and refrigerate for at least 1 hour or up to 24 hours.

Make the filling: Stir together poppy seeds, honey, and orange zest and juice in a small bowl. Let stand for about 15 minutes. Grate chocolate into shavings using the largest holes of a box grater. Set aside in a separate dish.

Roll dough to about a 16- x 11-inch (1/8-inch-thick) rectangle on a floured work surface. Using a small offset spatula or back of a spoon, spread a thin layer of poppy seed mixture over the dough, leaving a 1/2-inch border along the top long side of dough. (Filling is thick, so spread gently to avoid tearing the dough.) Sprinkle shaved chocolate over the poppy seed mixture.

Brush the long top dough border with water. Roll dough up into a tight log, starting at the bottom long end; pinch seam to seal. Cut log in half to yield 2 equal logs; wrap each log in plastic wrap, and freeze until firm enough to cut cleanly, about 20 minutes. (Alternatively, refrigerate logs overnight before slicing.)

Preheat oven to 350°F with racks in upper and lower third position. Line 2 baking sheets with parchment paper. Brush each log lightly with water, and roll in sparkling sugar, evenly coating the exterior of the logs.

Cut log into 1/4-inch-thick slices using a sharp knife. Arrange slices 1 1/2 inches apart on prepared baking sheets. Bake cookies on upper third and lower third racks, rotating baking sheets from top to bottom halfway through baking time, until cookie bottoms are light golden brown, 12 to 13 minutes. Cool on pans, about 15 minutes. Store in an airtight container at room temperature for up to 4 days.