In a large bowl, combine flour, sugar, baking powder and salt. Cut in butter until mixture resembles coarse crumbs. Crack an egg into a measuring cup and then fill with milk to make 1 cup. Stir in vanilla. Pour into crumb mixture and mix just until moistened. Spread into prepared pan. Sprinkle top with streusel. Bake 35-30 minutes or until knife comes out clean.
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