Melt butter and unsweetened chocolate together — on the stove over very low heat, stirring constantly, in a double-boiler or in the microwave in 30-second bursts, stirring between each — and taking the chocolate out when is not fully, melted. Off the heat, stir until it is.
Whisk sugars into melted butter and chocolate, then eggs, one at a time, then vanilla. Whisk in baking soda and salt. If your cocoa powder is lumpy, sift it right onto the surface of the batter, other wise, just add it to the bowl. Add flour, too, and then stir until just combined. Add chocolate chips and stir until combined.
Place bowl in the fridge for about 30 minutes, and up to a few days. Shorter than 30 minutes, you can bake it right away but they keep better shape once chilled and are easier to scoop. Longer than 30 minutes, they become difficult to scoop.
Heat oven to 350°F (175°C).
Scoop into about 1 1/2- to 2-tablespoon sized mounds and space evenly on parchment- or nonstick baking mat-lined baking sheets, allowing room for them to spread a little. Bake for 11 to 12 minutes, at which point they will still definitely look underbaked, but you should take them out for a fudgy-centered cookie. Let set on baking sheets for a few minutes before carefully transferring to a rack to cool the rest of the way before eating.
The cookie dough keeps in the fridge for up to a week and longer in the freezer.
