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

In a mixing bowl, combine softened unsalted butter and brown sugar until fluffy. Add vanilla extract and mix again.

Gradually add all-purpose flour, baking powder, salt, and oats, mixing until the mixture resembles a crumbly texture.

Press more than half of the crumble mixture into the bottom of the prepared baking pan and bake for about 12 minutes.

In another mixing bowl, whisk together sweetened condensed milk, lemon juice, and lemon zest until smooth.

Pour the lemon filling over the hot crust and sprinkle the remaining crumble mixture on top.

Return to the oven and bake for another 22-25 minutes until golden edges form.

Allow the bars to cool completely in the pan on a wire rack before slicing into squares.