Preheat oven to 375 degrees F. Line 2 large sheet pans with parchment paper.
Cream butter and sugars for 1 minute. Scrape sides of bowl. Continue beating for another minute. Scrape bowl again.
Add vanilla. Beat for 1 minute. Scrape sides of bowl.
Add egg yolks. Beat for 1 minute. Scrape sides of bowl.
Add flour, salt and baking soda. Beat 1 minute. Scrape sides of bowl and beat for another minute
Make a well in the middle of dough and pour in the melted chocolate. Gently fold the dough over with a spatula or knife, until you achieve a marbled look. Don't overmix the dough, or you'll have a chocolate cookie instead of a marbled cookie, which isn't a bad thing, it just won't look as cool.
Place course sugar in small, shallow bowl. Using a small cookie/ice cream scoop (mine is 1 1/2" in diameter) dipped in a mug of hot tap water, scoop balls of dough and drop two at a time into the course sugar. The hot tap water gives the dough balls a little tackiness and helps to get the course sugar to stick. Gently roll around in the sugar (the dough balls, that is.) Place balls of dough on parchment, leaving about 1 1/2" space around each. My half-sheet pans fit 12 cookies very comfortably.
Do not press the balls down. This will ensure a chewy middle.
Bake for 8 - 10 minutes, turning and reversing pans midway through baking. Resist the urge to bake your cookies longer, or they won't be very chewy.
Place pans on cooling racks. When cool, store cookies in air-tight containers.