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

Cream together the unsalted butter and brown sugar until smooth. Stir in vanilla extract, followed by flour, baking powder, salt, and oats.

Press slightly more than half of the mixture into the prepared pan. Bake for about 12 minutes until just golden.

In a separate bowl, whisk together sweetened condensed milk, lemon juice, and lemon zest until smooth.

Pour the lemon filling over the pre-baked crust and spread evenly. Sprinkle with the remaining crumble mixture.

Bake for an additional 22 to 25 minutes, until the edges are golden and the center is slightly soft.

Allow the bars to cool completely on a wire rack before slicing.