"Using up the last of my summer berries...."
INGREDIENTS
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1 (18 1/4 ounce) package yellow cake mix
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1/2 cup butter, softened
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1 large egg
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1 tablespoon grated lemon zest
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1 (14 ounce) can sweetened condensed milk
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1/2 cup fresh-squeezed lemon juice
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2 cups fresh blackberries (or frozen, thawed)