In a large bowl, combine softened butter (not melted, soften by leaving out at room temperature for several hours or briefly heating in the microwave), muscovado sugar, caster sugar, and fleur de sel. Mix until creamy and well combined.
Beat an egg in a separate container, measure out 35g, and add to the butter mixture along with the vanilla extract.
Sift together the flour and baking powder, then gradually add to the wet ingredients, mixing until just combined.
Fold in the chopped dark chocolate.
The dough will be sticky; chill in the refrigerator for at least 30 minutes to make it easier to handle.
Preheat the oven to 180°C (350°F).
Line a baking sheet with parchment paper. Form the dough into approximately 80g balls and place on the prepared sheet.
Bake for 13 minutes. Allow cookies to cool on the sheet for 15 minutes before transferring to a wire rack.
Optional: Press a few extra chocolate chunks into the tops of the cookies right after baking for added decadence.