In a small bowl, combine the flour, cocoa and baking powder; set aside. In a large bowl, beat the butter and sugars for a few minutes, until nice and light. Add the salt and vanilla and beat for 2 minutes more.
Add the flour mixture and stir by hand or beat on low speed just until it starts to come together; add the chocolate and stir or beat just until everything is incorporated.
Turn the dough out onto the counter, gather it into a ball or rough log, and divide it into three pieces. Shape each piece into a log that's roughly 1 1/2 iinches in diameter. Wrap in plastic wrap and refrigerate for at least 3 hours, or freeze for up to 4 months. (You don’t even need to defrost it before baking — just slice frozen logs and bake as you normally would.)
Place a rack in middle of the oven and preheat it to 325°F. Cut the logs into 1/2-inch slices with a sharp knife (if they crack, just squeeze them back together) and arrange onto a baking sheet (no need to spray or line it), spacing them about an inch apart.
Bake for 12 minutes - they won't be firm, and it will be hard to tell if they're done, because they're so dark. Don't sweat it. Let the cookies cool for a few minutes on the sheet, then transfer them to a wire rack. Eat warm, or cool completely.
Bake the cookies one sheet at a time for 12 minutes — they won’t look done, nor will they be firm, but that’s just the way they should be. Transfer the baking sheet to a cooling rack and let the cookies rest until they are only just warm, at which point you can serve them or let them reach room temperature.