In a large bowl, cream the butter with both sugars until smooth and cohesive. Add the egg, egg yolk, and vanilla, mixing just until incorporated. Add the flour, oats, baking soda, and fine salt, stirring until no dry spots remain. Fold in the chopped dark chocolate.
Portion the dough into balls, about 2 tablespoons each, and arrange on a parchment-lined baking sheet. Freeze for 60 minutes.
Heat the oven to 340°F/170°C.
Arrange the cookies on a lined baking sheet, spacing them well apart. Bake until the edges are set and lightly golden while the centers remain soft, 8 to 12 minutes.
Remove from the oven while slightly underbaked; the cookies will continue to set as they cool. Sprinkle lightly with coarse salt and cool completely.