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

In a mixing bowl, beat together the softened unsalted butter and brown sugar until smooth and creamy, about 2-3 minutes. Add the vanilla extract and blend.

In a separate bowl, whisk together the all-purpose flour, baking powder, salt, and oats. Gradually fold this mixture into the wet ingredients until crumbly.

Press slightly more than half of the crumble mixture into the bottom of the prepared pan and bake for 12 minutes until edges are golden.

While the crust bakes, mix together the sweetened condensed milk, lemon juice, and lemon zest until smooth.

Pour the lemon filling over the baked crust and evenly spread it. Sprinkle the remaining crumble mixture on top.

Return to the oven and bake for an additional 22–25 minutes, until edges are golden and center is set.

Allow to cool completely in the pan before lifting the bars out using the parchment. Cut into squares.