Preheat the oven to 350ºF. Heat semisweet chocolate chips and butter in a microwave-safe bowl in 20-second increments, stirring between each, until almost melted; do not overheat. In another bowl, whisk together flour, baking powder, and salt.
In a mixing bowl, beat eggs, brown sugar, and vanilla on high speed until light and fluffy. Reduce speed to low; beat in melted chocolate. Mix flour mixture until just combined. Stir in 60% bittersweet chocolate baking chips.
Drop by heaping tablespoons of dough 2 to 3 inches apart onto ungreased baking sheets. Bake until cookies are shiny and crackly, yet soft in centers, 12 to 15 minutes. Cool on baking sheets 10 minutes; then, with a thin metal spatula, transfer to racks to cool completely.
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