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Preheat oven to 350F.
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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.
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Press half of oat crumb mixture into the bottom of an 8x8-inch pan.
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In a separate mixing bowl, whisk together condensed milk, lemon juice, and lemon zest. Spread onto the bottom layer of the crumb mixture.
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Take the remaining crumb mixture and knead it with your hands to make it crumbly and chunky. Sprinkle this over the lemon mixture.
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Bake for 20 to 25 minutes, or until golden brown.
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Cool for about 30 minutes before cutting. Refrigerate for at least a couple hours before serving.