Place the 54 percent bittersweet chocolate and unsweetened chocolate in a heatproof bowl and microwave on high for two 30-second intervals, stirring after each interval. If still not smooth, heat for an additional 10 seconds at a time and stir until it is. Set aside until cooled to 90°F, about 15 minutes.
Whisk the flour, baking powder, and salt together in a small bowl, then transfer to a paper plate. Then, in the same bowl, whisk the eggs and vanilla together and set aside.
Mount the paddle attachment on your stand mixer and cream the butter and sugar on medium speed until the mixture looks like wet sand, about 2 minutes.
Reduce speed to low, and slowly add the egg mixture until fully incorporated. Pour in the melted chocolate and mix to combine, stopping to scrape down the sides of the bowl as needed. Then, with the mixer still on low, work in the flour mixture. When the batter seems homogenized, mix in the 70 percent bittersweet chocolate, milk chocolate, and cocoa nibs.
Cover the bowl with plastic wrap and refrigerate for 45 minutes.
Heat the oven to 350°F. Line two half sheet pans with parchment paper.
Scoop the dough onto the prepared pans using a 1 1/4-inch-diameter disher or ice cream scoop, placing the mounds 2 inches apart for 12 cookies per pan. Bake for 8 to 9 minutes, rotating the pans after 5 minutes. The cookies may still look a bit wet when they come out, but that's okay.
Cool the cookies on the pan for 2 minutes, then slide the parchment paper to a wire rack to let the cookies cool completely. Repeat steps 7 and 8 with remaining dough.