Preheat oven to 350 degrees. Heat chopped bittersweet chocolate and butter in a microwave-safe bowl in 20-second increments, stirring in between, until almost melted; do not overheat. Alternatively, you can do this in a double boiler or saucepan over low heat.
Mix together together flour, baking powder, and salt and set aside.
With an electric mixer, beat egg, brown sugar, and vanilla on high speed until light and fluffy. Reduce speed to low; beat in melted chocolate. Add flour mixture and stir until combined. Makes sure batter is not still warm from the melted chocolate, then stir in cut up chocolate bar.
Drop heaping tablespoons of dough 2 to 3 inches apart onto baking sheets. Bake, rotating sheets halfway through, until cookies are shiny and crackly yet soft in centers, 12 to 15 minutes. Cool on sheets 10 minutes; with a thin metal spatula, transfer to racks to cool completely.