Prep: Preheat your oven to 350°F (175°C). Line an 8×8 or 9×9 inch baking pan with parchment paper, leaving an overhang for easy removal.

Make the Oat Mixture: In a large bowl, whisk together the melted butter and brown sugar. Add the oats, flour, baking soda, and salt. Mix until a cohesive, crumbly dough forms.

Press the Base: Reserve about 1 heaping cup of the oat mixture for the topping. Press the remaining mixture firmly and evenly into the bottom of your prepared pan. Use the bottom of a measuring cup to pack it down tightly.

Make the Lemon Filling: In a medium bowl, whisk together the sweetened condensed milk, fresh lemon juice, lemon zest, egg yolk, and salt until smooth and well combined.

Assemble: Pour the lemon filling evenly over the prepared crust. Crumble the reserved oat mixture evenly over the top of the lemon layer.

Bake: Bake for 30-35 minutes, or until the edges are golden brown and the topping is set. The center may still look slightly soft but will firm up as it cools.

Cool Completely: Allow the pan to cool completely on a wire rack. For cleanest cuts, refrigerate the bars for at least 2-3 hours or overnight.

Serve: Use the parchment paper to lift the entire bar block out of the pan. Place on a cutting board, slice into squares, and dust with powdered sugar if desired.