Pre-heat the oven to 325ºF. Line an 8 by 8 pan with parchment paper.

To a medium-sized bowl add softened/melty butter and granulated sugar. Mix with a hand mixer for about a minute, scraping down the sides as needed until light and fluffy. Add the sourdough discard and mix again until the discard is completely incorporated into the butter/sugar mixture. Add vanilla extract and salt. Mix again. Add the flour and mix until soft chunks of dough form.

Pour the dough into the lined baking pan. Use your fingers to press the mixture into an even layer on the bottom of the pan. Prick the shortbread crust with a fork and bake for 25-30 minutes until the edges start to brown and the shortbread crust is baked through.

While the crust bakes, whisk together the eggs and sugar in a small bowl. Add the sourdough discard and whisk completely until smooth. This is an important step - the sourdough discard should be completely broken down and incorporated before adding the rest of the ingredients. Zest in a lemon and pour in the freshly squeezed lemon juice. Whisk to combine. Pour in the milk and whisk.

As soon as the shortbread crust comes out of the oven, pour the lemon bar filling over the top of the bars. It is normal for this filling to be very liquidy when you pour it in. Don't panic! Bake the bars for 30-35 minutes until the middle of the bars is set with no jiggles or wobbles in the middle.

Let bars cool for about 1 hour at room temperature. Then place the entire pan of bars into the refrigerator to chill for about 2 hours or overnight. Once bars are chilled, they can be sliced into squares and dusted with powdered sugar - right before serving. Enjoy!