Preheat the oven to 325° F. Position racks in the upper and lower thirds of the oven.
Combine the flour and baking soda in a small bowl and mix together thoroughly with a whisk.
In a large bowl, whisk together the melted butter, oats, sugars, corn syrup, milk, and salt.
Mix in the flour mixture. If the batter is warm from the butter, let it cool before adding the chocolate. Stir in the chocolate chunks.
Let the dough rest for at least several hours at room temperature or (better still) overnight in the fridge. The rest produces especially crisp and extra-flavorful cookies!
Divide the dough into 15 equal pieces (each a scant 1/4 cup or about 50 grams). Lay out 3 sheets of aluminum foil, cut to fit your cookie sheets, on the counter. Arrange 5 pieces of dough (4 in a square and 1 in the center) well apart on each sheet of foil, remembering that the cookies will spread to 5 inches. With your fingers, flatten each piece of dough until it is about 3 1/2 inches in diameter (use a measuring stick!). Slide two of the sheets onto baking sheets.
Bake for 20 to 25 minutes, until the cookies are thin and very brown. If they are too pale, they will not be crispy when cool. Rotate the pans from top to bottom and front to back halfway through the baking time to ensure even baking. Slide the foil with the cookies onto racks to cool completely before removing the cookies from the foil.
Repeat with the third batch—you can even slide the next foil with the cookie dough onto an already-hot baking sheet as long as you put the pan in the oven immediately.
Cool the cookies completely before stacking or storing. They may be kept in an airtight container for at least 3 days.
My career was sparked by a single bite of a chocolate truffle, made by my Paris landlady in 1972. I returned home to open this country’s first chocolate bakery and dessert shop, Cocolat, and I am often “blamed” for introducing chocolate truffles to America. Today I am the James Beard Foundation and IACP award-winning author of ten cookbooks, teach a chocolate dessert class on Craftsy.com, and work with some of the world’s best chocolate companies. In 2018, I won the IACP Award for Best Food-Focused Column (this one!).