Preheat your oven to 350°F (177°C) and prepare an 8-inch square baking pan by lining it with parchment paper.

In a large mixing bowl, beat together the butter and brown sugar until creamy. Add the vanilla extract, flour, baking powder, salt, and oats. Stir until crumbly.

Press slightly more than half of the mixture into the bottom of the pan. Bake for 12 minutes or until lightly golden. Let cool slightly.

Whisk together the condensed milk, lemon juice, and lemon zest until smooth.

Pour the lemon filling over the cooled crust. Sprinkle the remaining crumble mixture on top.

Bake for an additional 22–25 minutes until golden around the edges and set in the center.

Cool completely on a wire rack before lifting out and slicing into squares.