Begin by prepping the dark chocolate. Roughly two dark chocolate bars. Set aside ½ cup (for melting) and reserve the remaining chocolate (for topping later).
Next, melt the butter and chocolate. In a small pot over medium heat, melt the butter. Remove from heat, then add the ½ cup chopped chocolate you set aside and stir until smooth and fully melted. Whisk in the cocoa powder until no clumps remain. Transfer the mixture to a mixing bowl and let it cool completely.
While the butter and chocolate are cooling, prepare the dry ingredients. In a small bowl, whisk together the flour, baking soda, and salt. Set aside.
Next, add the brown sugar and granulated sugar to the cooled butter and chocolate mixture. Whisk together until smooth.
Whisk in the eggs one at a time, making sure each is fully incorporated before adding the next. Then add in the vanilla extract.
Add the flour mixture to the wet ingredients and mix until combined.
Scoop the dough with a 3-oz cookie scoop onto a parchment-lined baking sheet. Roll each portion into a ball, then gently flatten the tops. Cover the pan with plastic wrap and refrigerate for at least 30 minutes to firm up.
While the dough chills, preheat the oven to 350°F. Arrange the cookies on a parchment-lined baking sheet, spacing them about 1 inch apart. Bake for 12–14 minutes, until set and just beginning to crack around the edges. Let the cookies cool on the pan for 5 minutes, then top with the reserved ½ cup chopped dark chocolate while still slightly warm. Allow to cool completely before enjoying!