Preheat the Oven: Preheat the oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper, leaving enough overhang to lift the bars out after baking.
Melt the Butter: Place butter in a microwave-safe bowl. Microwave in 20-second intervals until melted, about 40-60 seconds. Alternatively, melt on the stovetop and pour into a large bowl.
Prepare the Crumble Mixture: To the melted butter, add brown sugar, baking powder, and 1/2 teaspoon of salt. Stir to combine. Add oats and flour, mixing until smooth. Set aside 1 cup of the mixture for the topping.
Bake the Crust: Press the remaining crumble mixture into the prepared pan. Bake for 12-14 minutes or until lightly golden. Remove from the oven and cool slightly.
Make the Lemon Filling: Zest and juice the lemons, then combine the zest, juice, sweetened condensed milk, and remaining salt in a bowl. Whisk until smooth.
Assemble and Bake: Pour the lemon filling over the crust. Sprinkle the reserved crumble mixture on top. Bake for 22-24 minutes or until the edges are golden. Allow the bars to cool completely, then refrigerate for 30 minutes if you prefer a firmer filling.
Serve: Lift the bars out of the pan using the parchment paper. Cut into 12 even pieces and serve.
