Melt the semisweet chocolate and butter together gently in short microwave intervals or in a heatproof bowl over simmering water. Stir until smooth and let the mixture cool slightly.

Whisk the granulated sugar, eggs, and vanilla until blended. Stir in the slightly cooled chocolate mixture.

In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually stir the dry ingredients into the chocolate mixture until a thick dough forms.

Chill the dough for 30 to 60 minutes, or until firm enough to scoop. Portion it into roughly 1-tablespoon balls and roll each ball generously in sparkling sugar.

Arrange the coated dough balls on parchment-lined baking sheets, leaving about 2 inches between cookies. Bake at 350°F (175°C) for 10 to 12 minutes, until the edges look set and the centers remain slightly soft.

Cool the cookies on the baking sheet for about 5 minutes before transferring them to a wire rack. They will firm as they cool while keeping their fudgy centers.