Melt the chopped semisweet chocolate, the bittersweet chocolate and the butter together in large heatproof bowl set over a pan of barely simmering water, making sure the water doesn’t touch the bottom of the bowl. Stir occasionally until smooth. Remove from heat and cool slightly. (Alternately you can melt the chocolate and butter in the microwave in short bursts, stirring after each one.)
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt until well combined.
In a large bowl, whisk together the brown sugar, sugar, eggs and vanilla extract until smooth and well combined, about 2-3 minutes. With a rubber spatula, stir in the melted chocolate mixture until combined.
Stir in the flour mixture just until combined. Fold in the chocolate chips. Refrigerate the cookie dough for about 10 minutes, just long enough to make the dough easier to work with.
Meanwhile, preheat the oven to 350ºF (180ºC). Line 2 large cookie sheets with parchment paper.
Roll 2-tablespoon (40g) size portions of cookie dough into balls. I used a #30 portion scoop.*** Place the dough balls on the cookie sheet, about 3 inches apart. Bake until the tops of the cookies are set and beginning to crack, about 9-11 minutes, being careful not to over bake. Cool the cookies on the cookie sheet for 1 minute, then transfer to a cooling rack. Sprinkle the warm cookies with sea salt flakes.