This is a decadent treat all chocolate-lovers will enjoy.
Whisk together flour, baking powder, and salt in a small bowl.
Place butter and bittersweet and unsweetened chocolates in a heatproof bowl. Set bowl over a pan of simmering water, and stir until smooth.
With a standing mixer fitted with the whisk attachment, beat eggs and sugar on medium-high speed until pale and fluffy. Beat in vanilla and chocolate-butter mixture. Reduce speed to low. Gradually add flour mixture, and beat until just combined. Stir in chocolate chips. Let dough stand for 30 minutes.
Preheat oven to 350 degrees. Drop teaspoons of dough onto parchment-lined baking sheets, spacing 1 inch apart. Bake until just set (interior will be fudgy), about 12 minutes. Let cool on sheets on wire racks.
Cookies can be stored in airtight containers for up to 3 days.