Add the unsalted butter, chopped unsweetened chocolate, and 1 cup of semi-sweet chocolate chips to a double boiler (a heatproof bowl set over a saucepan of simmering water). Stir continuously until the chocolate and butter have melted. Alternatively, you can melt the butter and chocolate in the microwave in 30-second intervals.
In a stand mixer with the whisk attachment, add the eggs, brown sugar, and granulated sugar. Whisk on high for 2-3 minutes, until the mixture becomes thick and lighter in color.
Slowly add the melted chocolate mixture to the egg and sugar mixture on low speed until combined. Sift the flour, baking powder, and salt into a medium bowl, then fold the dry ingredients into the wet ingredients by hand.
Gently fold in the remaining 1 cup of chocolate chips until evenly distributed in the dough.
Refrigerate the dough for at least 1 hour to firm it up.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone baking mat.
Scoop heaping tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes. For a softer, gooier center, bake for 10 minutes. The edges should be slightly crisp but still chewy.
Allow the cookies to cool for a couple of minutes on the baking sheet before transferring them to a cooling rack to cool completely.