Preheat oven to 350 degrees. Line 2 standard 12-cup muffin tins with paper liners. Whisk flour, baking powder, and salt.
Put butter and chocolate into a heatproof bowl set over a pan of simmering water; stir until smooth. Remove from heat, and let cool slightly.
With an electric mixer on medium speed, beat chocolate and sugar for 3 minutes. Add eggs, one at a time, and beat until combined. Beat in vanilla. Reduce speed to low. Add flour mixture; beat until just combined.
Divide batter evenly among muffin cups. Bake until set and a cake tester inserted into centers comes out clean, about 18 minutes. Transfer to wire racks; let cool completely.
Spoon icing over cupcakes, covering tops completely. Let set, about 1 hour. Decorate with sugared flowers. Cupcakes can be stored in single layers in airtight containers for up to 2 days.