Preheat your oven to 350°F (177°C) and prepare an 8-inch square baking pan by lining it with parchment paper, allowing some overhang for easy removal.

In a large mixing bowl or stand mixer, combine 7 tablespoons of softened unsalted butter with 3/4 cup of brown sugar. Beat until creamy, then add 1 teaspoon of vanilla extract and mix. Gradually incorporate 1 cup of all-purpose flour, 1 teaspoon of baking powder, 1/4 teaspoon of salt, and 1 cup of old-fashioned oats until crumbly.

Take more than half of the crumble mixture and press it into the bottom of the prepared pan to create a solid crust layer. Reserve the remaining crumble for topping.

Bake the crust in your preheated oven for 12 minutes or until lightly golden. Remove from the oven and let it cool slightly.

In a medium mixing bowl, whisk together 1 can of sweetened condensed milk, 6 tablespoons of lemon juice, and 1 tablespoon of lemon zest until smooth.

Pour the lemon filling over the warm crust, spreading evenly. Sprinkle the reserved crumble mixture on top.

Return the pan to the oven and bake for an additional 22–25 minutes. The edges should turn golden, and the center will be slightly jiggy.

Once baked, allow the bars to cool completely on a wire rack. Use the parchment overhang to lift the bars out and slice them into squares.