Step 1 In a large bowl, sift together the flour and baking soda. Whisk in the salt.
Step 2 In the bowl of a stand mixer, or in another large bowl using a hand mixer, beat the butter until pale and fluffy, 2 to 3 mintues. Beat in the powdered and muscovado sugars. Beat in the eggs, one at a time, until fully incorporated.
Step 3Scrape the seeds from the vanilla bean and beat in with the butter mix. Add the dry ingredients and slowly beat until a soft dough forms. Stir in the chocolate, careful not to overmix.
Step 4Cover the dough with plastic wrap and refrigerate until firm enough to scoop, at least 20 minutes, up to a day in advance (the cookies can also be divided and frozen on sheets before baking). Meanwhile, heat the oven to 350 degrees.
Step 5Shape the dough into 12 balls. Place the balls, 6 to a sheet, on parchment-lined baking sheets, flattening them slightly. Bake the trays, one at a time, until the cookies are set on the outside but still soft in the center (the cookies may look a bit underdone, but they will set up as they sit), 15 to 20 minutes. Rotate the cookies halfway through baking. Cool the cookies on the tray for 10 minutes, then carefully remove to a rack to cool completely.