Begin by deciding if you want walnuts in your cherry pie bars. (You do.) Preheat your oven to 375 degrees F. Spread 1/2 cup chopped walnuts over a dry baking sheet. Bake for about 5 minutes, stir, then continue baking in 2 minute increments, stirring every time, until the nuts are browned and quite fragrant. Remove from the oven and let cool.
Meanwhile, line a 9x13 inch pan with parchment paper, or grease with nonstick spray or butter. Set aside.
Make the shortbread crust. In a large bowl or stand mixer, add 2 cups of butter. Beat thoroughly for a couple minutes until the butter is smooth and there are no lumps. Be sure to scrape the sides and bottom of the bowl.
Add 1 cup sugar. Continue beating for another 2 minutes, until the butter is light and fluffy.
Add 1 teaspoon vanilla extract, 1 teaspoon almond extract, and 1 and 1/2 teaspoons kosher salt. Beat to combine.
Add 1 cup of flour, making sure you are spooning the flour into the measuring cup and leveling it off. Beat until combined. Add the remaining flour 1 cup at a time (total of 4 cups), beating each time. Make sure to scrape the sides and bottom of the bowl. If you are using a hand mixer, you might need to mix the last cup or two manually with a wooden spoon.
Add about 2/3 of the dough to the prepared pan. (I like to separate the dough into chunks and spread them across the pan to make it easier to press in.) Use your hands to press the dough flat into the bottom of the pan. If your hands stick to the dough, lightly wet your hands. Place the pan in the fridge for about 20 minutes. Cold shortbread dough going into a hot oven is what gives you that perfect flaky texture.
Add the completely cooled walnuts to the remaining 1/3 of the dough that's still in the mixing bowl. Mix the walnuts in, cover, and chill in the fridge while your crust bakes.
Bake the crust. After a 20 minute chill in the fridge, bake the crust in the oven at 375 degrees F for about 18-20 minutes. (If you are using a glass pan, let the cold pan rest on the counter for about 2-3 minutes before putting it in the hot oven; you don't want to shatter your pan.) You will know the crust is done baking when the edges look golden brown and it is not shiny in the center. Set aside to cool.
Lower the oven temperature to 350 degrees F.
Make the cherry filling. Meanwhile, prep your cherries. Use a cherry pitter or a straw (a metal straw would work best) to pit 2 and 1/2 pounds cherries, or about 8 cups of cherries. Leave them whole, no need to halve them. Add the cherries to a 3 quart pot.
Add 1 and 1/4 cups sugar, 1/4 cup lemon juice, 1 and 1/2 tablespoons cornstarch, and 1/4 teaspoon kosher salt. Stir together.
Turn the burner on to high heat and bring the mixture to a boil. Once boiling, lower the heat to maintain a strong bubbling simmer (Medium or medium high) and let boil for about 5-6 minutes, stirring occasionally. The cherries will soften a bit and the liquid will get thick. The mixture will foam and rise up a bit, this is normal.
Pour the hot cherries onto the baked shortbread crust.
Working quickly, add the remaining shortbread crust on top the cherries. You want your shortbread to stay as cool as possible, so don't take forever. I like to pinch the dough off with my fingers into pieces about the size of a quarter and scatter them over the top. It's okay if you can still see cherries through the topping. See photos. At this point, you could sprinkle a tablespoon or two of demerara sugar over the topping if you want an extra crunch.
Make sure your oven has been lowered to 350. Bake the cherry pie bars for about 25-30 minutes, until the top is lightly browned.
Remove from the oven and let cool completely on a wire rack. If you are in a hurry (I always am!) you can let the bars chill in the fridge or freezer for a while to speed up the cooling process. Or you could cut into them after setting up for 10 minutes and eat them hot and warm with vanilla ice cream, although they will be a mess. Isn't a mess worth it sometimes though?