Grease a 9-X-13-X-2-inch baking dish lightly with butter or baking spray. Heat the oven to 350℉.
Make the Caramel:
In a medium saucepan with sides at least 4 inches tall, melt the sugar over medium heat, nudging the dry patches of sugar into the melted areas with a wooden spoon, gently stirring as needed to prevent any areas from burning as the sugar caramelizes. Continue until the caramel reaches a medium amber color, like a hazelnut's shell. Remove the pan from the heat, and immediately add the butter and cream – be careful, the mixture will bubble up vigorously. If the caramel hardens, return to low heat, stirring, until smooth. Stir in the salt, and set aside. After cooling for at least 5 minutes, stir in the 3 tablespoons of flour.
Make the Cookie Layer:
Cream the butter, brown sugar, vanilla and salt together using an electric mixer, or mix by hand with a wooden spoon, until no butter streaks remain. Add the oats, flour and baking soda, and stir on low speed until no visible flour streaks remain and a buttery, thick, crumbly dough has formed.
Set aside 1 cup of the dough. Press the remaining crumbs into an even layer in the prepared pan. Wetting your fingers with a little water will make the sticky dough easier to handle. Bake for 14 to 18 minutes, until slightly puffed and the surface has lost its wet look around the edges. Remove from the oven, leaving the oven on. Let the parbaked dough rest for 10 minutes before proceeding. I use this time to do the dishes.
Scrape the caramel out of its pan and spread it gently over the cookie base. Scatter the nuts, if using, and chocolate chips over the caramel mixture. Top with chunks of the reserved dough. Bake for an additional 15 to 18 minutes, until the edges are bubbling and the dough chunks are a light golden brown. Cool to room temperature before refrigerating for at least an hour. When firm, cut into squares.