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

In a mixing bowl, beat the softened unsalted butter and brown sugar together until well combined and fluffy, about 2-3 minutes. Add the vanilla extract, followed by mixing in the all-purpose flour, baking powder, salt, and oats until the mixture resembles a crumbly texture.

Take slightly more than half of the crumble mixture and press it firmly into the bottom of the prepared baking pan. Bake this crust in the preheated oven for 12 minutes until it starts to set and lightly golden.

Let the crust cool slightly while preparing the lemon filling; the base will firm up as it cools.

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

Carefully pour the lemon filling over the cooled crust, spreading it evenly to the edges. Sprinkle the remaining crumble mixture over the filling.

Return the pan to the oven, baking for an additional 22-25 minutes or until the edges are golden brown and the center is set.

Allow the bars to cool completely in the pan on a wire rack, then lift out and slice into squares.