10 ounces semisweet chocolate, chopped into chunks – divided
2 tablespoons unsalted butter
1/3 cup all-purpose flour, spooned and leveled
1/4 teaspoon baking powder
1/4 teaspoon salt
1 large egg
1/3 cup + 1 tablespoon packed light-brown sugar
1/2 teaspoon pure vanilla extract
Preheat oven to 350 degrees. Heat 4 ounces of the chopped chocolate and butter in a microwave-safe bowl in 20-second increments, stirring in between, until almost melted; do not overheat.
In another bowl, whisk together flour, baking powder, and salt.
In a mixing bowl, beat egg, brown sugar, and vanilla on high speed until light and fluffy. Reduce speed to low; beat in melted chocolate. Mix in flour mixture until just combined. Stir in remaining chocolate chunks.
Drop heaping tablespoons of dough 2 to 3 inches apart onto parchment lined baking sheet. Bake 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.