Preheat the oven to 375 degrees.
For the crust, place the butter, granulated sugar, brown sugar, and vanilla in the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed for about 2-4 minutes. You want the mixture to be super light and fluffy which will make a much flakier crust.
While the butter mixture is getting nice and fluffy, sift the flour and salt together. Sifting the flour will make the crust nice and light and flakey.
Then, with the mixer on low, SLOWLY (so the flour doesn't go all over you and your counter) add flour to the butter-sugar mixture.
Beat until combined before adding more flour. And beat only until flour is just barely combined -- don't over mix.
Scatter two-thirds of the dough in clumps in a 9 x 13-inch baking pan. Lightly flour your hands to prevent the dough from sticking, and then gently press dough into the bottom and 1/2 inch up the sides. Be sure there are not holes or gaps. Leave the remaining 1/3 of the dough in the mixer -- you'll be finishing it in just a sec.
Refrigerate dough for 15 minutes.
Then bake for 18 to 20 minutes, until the crust is golden brown, and set aside to cool.
While the crust is baking, add the walnuts and cinnamon to the remaining dough in the mixer, and mix on low speed until the ingredients are just combined. Set aside.
Reduce the oven to 350 degrees.
For the filling, combine the 2 types of apples you're using along with the lemon juice in a very large bowl.
Stir in the granulated sugar, cinnamon, nutmeg and cardamom until all the ingredients are incorporated.
Melt the butter in a large skillet, add the apples, and simmer over medium to medium-low heat, stirring often, for 12 to 15 minutes, until the apples are tender, they begin to turn caramel brown and the liquid has mostly evaporated.
Taste the apples to see if they need any more sugar or spices --
Spread the apples evenly over the cooled crust, leaving a 1/2-inch border.
Pinch medium pieces of the remaining dough with your fingers and drop them evenly over the top of the apples. Don't worry about the dough covering all the apples because it won't. And that's ok! When the dough bakes it will spread out a bit with some of the apples peeking through.
Bake for 25 to 30 minutes, until the topping is golden brown.
Cool completely and cut into bars.