Preheat the oven to 350 degrees, with a rack in the center position. Line the baking pan with non-stick aluminum foil or regular aluminum foil, extending it several inches beyond the edge on two opposing sides to use as handles when removing the cookies. If using regular foil, grease it well.
In a mixing bowl, combine the flour, oats, brown sugar, and baking soda and stir/toss until well combined. Pour the melted butter over this and stir until all the dry ingredients are evenly moistened. Set aside a generous half cup of this mixture to use as a topping and press the rest in the prepared pan, making sure it's level and smooth. Bake until set and light golden, about 12-15 minutes in my oven. Remove to a rack to cool slightly (about 5-8 minutes) but leave the oven on.
Make the filling: In a mixing bowl (I wipe out the bowl I used earlier with a paper towel and use it again) combine the white chocolate chips, toasted pecan pieces, and toffee bits. Sprinkle the mixture over the warm base, leveling it as you do. Combine the caramel sauce and three tablespoons of flour in a small bowl (I reuse the bowl in which I melted the butter after wiping it out with a paper towel) and stir or whisk vigorously until no flour is visible. Drizzle the sauce evenly over the white chocolate-nut-toffee bit mixture. Make a top layer by sprinkling the 1/2 cup of the flour-oat combination evenly all over the sauce. Return the pan to the oven and bake until the topping is set and turns light golden and the caramel bubbles at the sides of the pan. While the flour-oat topping will be crunchy, the batter below it will still be slightly soft when it's done. Start checking at about 20 minutes in the oven. Don't overbake or you'll lose the wonderful, gooey texture.
Remove the pan from the oven and let cool on a rack until the uncut cookies are firm. Run a knife or offset spatula around the edge of the pan to loosen any caramel that sticks to the sides. Using the foil handles, carefully lift the block of cookies from the pan and, still in the foil, set them on a rack to cool completely. Cut into bars and serve or, if you are freezing them, leave them in a block, wrap them airtight, and cut into squares when they have defrosted.
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