Preheat the oven to 175°C (or 350 degrees F). Line 2 large baking trays with baking parchment.
In a large bowl, combine the butter and sugars. Using a wooden spoon, cream the mixture until light and fluffy, for about 3 minutes.
Lightly beat the egg and vanilla together in a small bowl or custard cup. Add to the butter mixture a little at a time, stirring until well combined. If you are using large eggs, don't blend the vanilla with the egg. Add the vanilla straight to the batter and only use 42 to 45 grams of the egg.
Mix together the flour, cocoa powder, baking soda and salt until very well combined and there are no lumps. If necessary, sift. Add to the butter mixture and stir to make a soft dough. Stir in all the chips (or chunks)
Using an ice cream scoop or cookie scoop or spoon, divide the mixture into 16 balls. Subway cookies weigh 1.6 oz, so you may want to weigh. Roll into balls and place onto the lined baking trays, spaced at least 2 ½ inches apart.
Bake for 10-12 minutes until well spread and just set on the surface. The cookies will still be quite soft, but will firm up as they cool. Leave on the tray for 5 minutes, then carefully transfer to a cooling rack to cool completely. Store cookies in an airtight container for up to 7 days.