Preheat oven to 350F.

Beat butter and brown sugar together until light and fluffy. Sift together flour, salt, and baking powder. Add oats and flour to butter/sugar mixture and mix to combine.

Press half of oat crumb mixture into the bottom of an 8x8-inch pan.

In a separate mixing bowl, whisk together condensed milk, lemon juice, and lemon zest. Spread onto the bottom layer of the crumb mixture.

Take the remaining crumb mixture and knead it with your hands to make it crumbly and chunky. Sprinkle this over the lemon mixture.

Bake for 20 to 25 minutes, or until golden brown.

Cool for about 30 minutes before cutting. Refrigerate for at least a couple hours before serving.