In a small saucepan over medium heat, combine the brown sugar with water. Cook, stirring with a heat-resistant spatula, until dark amber, for 10 to 12 minutes. Slowly whisk in heavy cream, unsalted butter, and kosher salt until smooth. Remove from heat and stir in the vanilla extract.

Lightly grease a 9 x 13-inch baking pan and line it with parchment paper long enough to overhang the edges.  Put the butter, granulated sugar, brown sugar, salt, and vanilla in the bowl of a stand mixer fitted with the paddle attachment and beat on medium speed until light and creamy, about 2 minutes. Sift the flour with the mixer on low and slowly add it to the butter-sugar mixture, beating until combined being careful not to overmix; the dough will be crumbly.

Scatter two-thirds of the dough in clumps in a 9 x 13-inch baking pan and press lightly with floured hands on the bottom and ½ inch up the sides. Refrigerate for 20 minutes. Meanwhile, preheat the oven to 375 degrees. Bake until the crust is golden brown, about 20 minutes. Set aside to cool.

Put the mixing bowl with the remaining dough back on the mixer, add the chopped walnuts and cinnamon, and mix on low speed to combine. Set aside.

Spread the apples evenly over the crust, leaving a ½-inch border. Crumble medium pieces of the remaining dough with your fingers and drop them evenly on top of the apples (they will not be covered). Bake for 30 minutes until the topping is browned. Cool completely in the pan and refrigerate for a few hours or overnight to firm up.

Once chilled, lift the parchment paper to release the bars from the pan. Drizzle evenly with the salted caramel. Using a sharp knife, cut into squares and serve with vanilla ice cream or whipped cream.