Preheat oven to 350 degrees. Heat chopped chocolate and butter in a microwave-safe bowl in 20-second increments, stirring between each, until almost melted; do not overheat. Remove; stir until melted and smooth. (Alternatively, place chocolate and butter in a large heatproof bowl set over (not in) a pan of simmering water; stir until melted and smooth) In another bowl, whisk together flour, baking powder and salt.
With an electric mixer, beat eggs, brown sugar, and vanilla on high speed until light and fluffy. Reduce speed to low; beat in chocolate mixture. Mix in dry ingredients just until combined. Fold in chocolate chunks.
Drop by tablespoons of dough onto baking sheets, 2 to 3 inches apart. Bake, rotating sheets halfway through until cookies are shiny and crackly, yet still soft in centre, 12 to 15 minutes. Cool on sheets 10 minutes; Transfer to wire racks to cool completely.