Melt the butter in a light-coloured pan over medium heat. Let it foam, then crackle, swirling now and then, for 5 to 7 minutes, until brown specks appear on the bottom and it smells nutty. Pour it into a large bowl, scraping in all the brown bits, and cool for about 10 minutes until warm, not hot.
Whisk both sugars into the butter until smooth and glossy.
Add the egg and vanilla and whisk for about a minute, until slightly lighter in colour.
Fold in the flour, baking soda and salt with a spatula until no dry flour is visible. If the dough looks crumbly, add 1 teaspoon of milk.
Stir in the chopped chocolate and pecans.
Cover and chill for 30 to 45 minutes. Meanwhile, preheat the oven to 180°C (350°F) and line two baking trays.
Roll the dough into 12 to 14 balls (about 45 g each) and space them well apart on the trays.
Bake one tray at a time for 10 to 13 minutes, until the edges are golden and the centres still look slightly underdone.
Right away, press extra chocolate chunks and a pecan half into the top of each cookie and sprinkle with flaky salt.
Cool on the tray for 10 minutes, then move to a rack.
