Line baking sheets with parchment paper. Set aside.

In a medium bowl, beat butter and brown sugar at medium-high speed with a mixer until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla.

In another medium bowl, sift together flour, cocoa, salt, and baking powder. Add flour mixture to butter mixture in thirds, beating at low speed until combined. Turn dough out onto a lightly floured surface. Shape into a disk. Cover with plastic wrap and refrigerate 2 hours.

Preheat oven to 350°. Scoop dough into mounds (approximately 2 tablespoons each). Using your hands, roll into balls. Sprinkle work surface with sugar. Using a rolling pin, flatten balls into 4-inch rounds. Place on prepared pans. Refrigerate 15 minutes.

Spoon approximately 1 tablespoon chocolate-hazelnut spread into center of each round, leaving a 1-inch border. Gather edges of dough, pinching together to enclose filling. Place on pan, pinched side down. Sprinkle with sparkling sugar.

Bake until firm, 12 to 15 minutes. Let cool on pans. Store in an airtight container up to 3 days.