Preheat oven to 350°F. Line a 9×9-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.

In a bowl, whisk together flour, baking soda, salt, and cinnamon.

In a separate large bowl, whisk melted butter with brown sugar and granulated sugar until smooth and glossy.

Whisk in egg, egg yolk, and vanilla until fully combined, then stir in sourdough discard until smooth.

Add dry ingredients to wet ingredients and mix just until combined.

Fold in the chocolate.

Press dough evenly into the prepared pan. Press extra chocolate into the top.

Bake for 25–34 minutes, checking early if your oven runs hot. If the top begins browning too quickly, loosely tent the pan with foil for the remaining bake time.

Bars are done when edges are golden, and the center looks just set and no longer glossy. A toothpick inserted into the center should come out with moist crumbs.

Sprinkle lightly with flaky salt.

Cool completely in the pan before lifting out and slicing.