Sift the flours, bicarbonate of soda, baking powder and salt into a bowl. Set aside.
Using a mixer fitted with paddle attachment, cream the butter and sugars together until very light – about 5 minutes. Add the eggs, one at a time, mixing well after each addition.
Stir in the vanilla. Reduce the speed to low, add the dry ingredients and mix gently until combined – just 5 to 10 seconds. Drop the chocolate pieces in and incorporate them without breaking them.
Wrap clingfilm around the dough and refrigerate for 24-36 hours (the closer to 36 the better.) The dough may be used in batches, and can be refrigerated for up to 72 hours.
Preheat the oven to 180C/350F/gas mark 4. Line a baking sheet with parchment paper or a nonstick baking mat. Set aside.
Scoop six mounds of dough, the size of generous golf balls, on to the baking sheet, making sure to turn horizontally any chocolate pieces that are poking up: it will make for a more attractive cookie.
Sprinkle lightly with sea salt and bake until golden brown but still soft – 18-20 minutes. Transfer the sheet to a wire rack for 10 minutes, then slip the cookies on to another rack to cool a bit more.
Repeat with the remaining dough, or reserve dough, refrigerated, for baking remaining batches the next day. Eat warm.