In a medium bowl, combine the flour, baking soda, and salt with a fork or whisk.
In the bowl of a mixer or by hand, beat the butter and sugars until smooth and fluffy.
Beat in the vanilla just to combine.
On low speed, beat in two of the yolks, followed by half of the flour mixture until combined.
Repeat with the other two yolks and then the rest of the flour mixture until just combined.
Transfer the cookie dough to a lightly floured work surface.
With your hands, divide the dough into 3 equal balls.
Flatten each ball and press it into a 4x6 inch (10.2x15.3 cm) rectangle.
Wrap each rectangle in plastic and refrigerate for 30-60 minutes.
Heat the oven to 375 while the dough is chilling.
Also, while the dough is chilling, chop your 10oz of chocolate roughly, dividing into two-4oz and one- 2oz group. The 2oz group should then be a bit more finely chopped than the other 8oz.
Flour a work surface or a large piece of parchment or wax paper [I use parchment].
Place one rectangle of dough on the floured surface or paper.
Sprinkle the rectangle with 4oz of the chocolate.
Top this with another rectangle of dough.
Sprinkle with another 4oz of the chocolate.
Cover with the last rectangle of dough, reserving the last 2oz of the finer-chopped chocolate.
Sprinkle the top layer of dough with a bit of flour and cover with a second piece of parchment or wax paper [or just use a floured rolling pin in step 19, below].
With a rolling pin, flatten this layered dough into a 9 x 6 inch [23x15.25cm] rectangle that is almost 1.5 inches [3.8cm] tall.
Remove the floured parchment or wax paper, if using.
Sprinkle evenly with the remaining 2oz of more finely chopped chocolate.
Recover with the parchment or wax paper, if using, and gently roll the top, just enough to press the chocolate in a wee bit.
With a long, sharp knife, cut the rectangle into 24 1.5-inch [5-cm] squares.
Place the cookie squares onto parchment- or silpat-lined baking sheets, 2-3 inches apart.
Bake the cookies, one tray at a time, for 12-14 minutes, rotating after 6 minutes cooking time. Cookies will be light brown and set.
Cool completely on wire racks.