Add the butter and both types of sugar into a bowl of a standing mixer fitted with paddle attachment (or a handheld electric mixer with standard beaters) and mix on medium- high speed until light, pale and fluffy. This should take about 3 minutes.
Add the egg and vanilla and beat all together. Scrape the bottom of the bowl to make sure all the ingredients are combined nicely.
In a large bowl, mix together both types of flour, baking soda and salt. Turn the speed on your mixer to low, and slowly, start adding the flour mix. Mix until JUST combined. Do not overmix! You should have few strokes of flour remaining, that is absolutely fine.
Remove the bowl from the mixer, and using a rubber spatula or a wooden spoon, mix in the chocolate/ chocolate chips. Mix it all together until the chocolate is nicely and evenly distributed in the dough. Cover the dough with some kitchen wrap and transfer to the fridge to chill for minimum 2 hours (or overnight for best results!).
When ready to bake, heat the oven to 170 C (fan). Line 2 baking trays with baking parchment. Using a cookie scoop or a measuring spoon, scoop the cookie dough and arrange on baking trays, leaving plenty of space between them (roughly about 5 cm).
Bake 1 tray at a time, in the middle shelf of the oven for about 10 minutes. Make sure to rotate the baking sheet half way through the baking. The cookies should be slightly golden brown around the edges, but the middle should be still a bit soft. Take them out of the oven, sprinkle with sea salt flakes while they are still warm, then and allow them to cool for about 5 minutes before transferring to a wire rack to cool completely.
make sure your ingredients are at room temperature
if you don't have any bread flour, simply replace it with plain flour
use different types and sizes of chocolate
chill cookie dough for at least couple of hours, but ideally overnight
make sure to bake one tray at a time, in the middle shelf of the oven
the cookies are ready when they still look a bit undone but the edges are turning golden brown
sprinkle some sea salt flakes on the cookies while they are still warm
you can freeze the cookie dough for up to 3 months