To start, you need to prepare the buttery shortbread crust that makes the base of the bars. Preheat the oven to 350°F (175°C). Line a 9×9 pan with parchment, leaving overhang. Don't skip this step, it's key to removal of the bars later. Without the parchment they'll stick the the pan!
You can use a food processor to make the short bread, or mix it by hand.
Combine the flour, sugar, and salt together either in a bowl if mixing by hand or in a food processor by pulsing together.
1/3 cup (65g) granulated sugar, 1 3/4 cups (220g) all-purpose flour, 1/4 tsp fine sea salt
Cut the cold butter into 1/4 inch cubes, and add to the dried ingredients. If using a food processor, pulse the butter into the dry ingredients just until coarse pea-sized crumbs form. If you're mixing this dough by hand, use a pastry cutter or fork to break up the butter to form coarse crumbs.
10 tbsp (140g) unsalted butter
Add the sourdough discard to the dough, and mix either in the food processor with gentle pulses until a dough begins to form or by hand until the dough starts to come together.
1/4 cup (60g) sourdough discard
If the dough is very dry and needs help coming together, don't over work! Instead, add ice water 1 teaspoon at a time until the dough roughly holds together.
Once the dough starts to form, transfer to the pan lined with parchment and press the dough firmly and evenly into the pan. If needed, you can cover the shortbread crust with more parchment paper and press with a measuring cup to help smooth the shortbread out to make it even.
Dock the bottom with a fork all over, by making small holes all over the bottom.
Optional Overnight Fermentation: Cover the shortbread crust with plastic, and refrigerate overnight. This allows the sourdough discard to work to break down the gluten in the flour. Remove from the refrigerator and bake the next day as directed below.
Bake the crust for 20 minutes until lightly golden at the edges. Remove from the oven when the shortbread is done, but don't turn off the oven! We are going to make the pecan filling while the crust bakes and return the bars back to the hot oven once we add the caramel pecan layer.