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

Cream together the softened unsalted butter and brown sugar until well combined and creamy. Add the vanilla extract and mix until blended.

Combine the all-purpose flour, baking powder, salt, and old-fashioned oats into the butter and sugar mixture, stirring until crumbly. Reserve some for the topping.

Press the larger portion of the crumb mixture into the bottom of your lined baking pan to form an even crust and bake for 12 minutes.

Whisk together the sweetened condensed milk, lemon juice, and lemon zest in a separate bowl until smooth.

Pour the lemon filling over the pre-baked crust and sprinkle the reserved crumb mixture on top.

Bake for an additional 22-25 minutes, until edges are golden and center is set.

Let cool on a wire rack and slice into squares once completely cooled.