Cut two pieces of parchment paper, about 8 by 15 inches, to create a sling for an 8-inch baking pan. Or you can use precut parchment paper like I’m using here. Set aside.

To make the dough: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, powdered sugar, vanilla, and lemon zest together until light and fluffy, about 3 minutes.

Add the sourdough discard and mix until well combined.

Press ¾ of the dough (you can just eyeball this!) into the bottom of the prepared pan, making sure to press it into the corners.

Wrap the remaining dough in a piece of plastic wrap. Refrigerate the pan and the remaining portion of dough (used for the topping) for at least 30 minutes.

Preheat the oven to 350°F.

To make the filling: While the dough chills, add the blueberries, cane sugar, cornstarch, and lemon juice to a small saucepan and stir together. Cook over medium/high heat until the blueberries begin to break down. Once the mixture boils, turn the heat down to low/medium and cook until the filling has thickened, about 15 minutes. Stir often to prevent the filling from burning.

Spread the warm blueberry filling evenly over the shortbread dough, making sure to get the filling into the corners.

Bake the bars for 40 to 45 minutes or until the shortbread topping looks lightly browned.

Allow the bars to cool completely, then sift powdered sugar on top. Cut into 16 bars (4x4).