Make the Salted Caramel:
Prepare all the caramel ingredients in advance, ensuring the heavy cream is at room temperature.
In a heavy-bottomed saucepan, combine water and sugar, stirring gently to mix evenly. Avoid splashing sugar onto the sides of the pot.
Over medium heat, let the sugar dissolve without stirring. Once it turns clear and bubbling, monitor closely as it transforms from clear to golden, then amber.
When the sugar reaches an amber color, remove it from heat immediately and whisk in the butter.
Slowly whisk in the heavy cream until smooth, followed by the vanilla and kosher salt.
Return the caramel to medium heat for 2 minutes, then whisk in the flour and set the caramel aside.
Prepare the Cookie Dough:
Preheat the oven to 350°F (175°C) and line an 8×8-inch baking dish with parchment paper, greasing the sides.
In a medium bowl, whisk together the oats, flour, baking powder, and salt.
In a large bowl, mix together the butter and dark brown sugar until smooth. Add the egg, egg yolk, and vanilla, and mix well.
Add the dry ingredients, mixing until just combined, then fold in the semisweet chocolate chips.
Assemble the Bars:
Spread ⅔ of the cookie dough evenly in the prepared baking dish, then sprinkle with the chopped nuts.
Bake for 8 minutes, placing the remaining dough in the refrigerator.
Remove the dish from the oven, top the crust with an even layer of dark chocolate chips, and pour over the caramel.
Drop random blobs of the remaining cookie dough on top.
Bake for 20-25 minutes or until lightly golden.
Cool and Serve:
Allow the bars to cool at room temperature for 30 minutes, then refrigerate for an hour.
Use a knife to release the bars from the pan if necessary, sprinkle with flaky sea salt, and slice into squares.