Preheat a fan-forced oven to 150C / 300F (170C / 340F for no fan). Line three large baking sheets with baking paper. I used half sheet baking trays. Set aside.
Combine dry ingredients - In a large mixing bowl, whisk together the bread flour, cocoa powder, bicarb soda, salt, and half of the white chocolate chips. Set aside.
Cream butter and sugar - In a separate large bowl, add the brown sugar, caster sugar, and softened butter (ensure it's soft but not melted). Beat with an electric mixer or stand mixer fitted with a paddle attachment for 2-3 minutes, until pale and creamy.
Add egg and vanilla - Stop the mixer and scrape down the bowl. Add the egg and vanilla extract, then mix until fully combined.
Add remaining ingredients - Stop the mixer and scrape down the bowl. Add the dry ingredients and use a spatula to mix until a dough forms with no visible dry ingredients.
Bake - Use a 2-tbsp cookie scoop to portion the dough, rolling it into 14 balls (6 per baking sheet). Bake one tray at a time on the middle rack for 10-12 minutes. The cookies will be very soft when removed from the oven.
How to get perfectly round cookies - As soon as the cookies come out of the oven, add the chopped chocolate so it melts and sticks to the tops. Then, use a round cookie cutter slightly larger than the cookies to gently shape them. Let the cookies cool completely before serving. Continue baking the remaining batches.