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Preheat oven to 350°F.
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Lightly coat a square baking dish with non-stick cooking spray.
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Cream 1/2 cup butter and granulated sugar until light and fluffy.
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Beat in eggs.
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Mix in vanilla extract.
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Combine 2 cups flour, 2 teaspoons cinnamon and baking powder. When well blended, alternately stir into the creamed mixture along with the buttermilk.
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Gently fold in blueberries but do not mix or batter will turn gray.
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Scrape batter into prepared cake pan, smoothing top with a spatula.
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Place oatmeal, brown sugar and remaining butter, flour and cinnamon into a small mixing bowl. Using your finger tips, blend together to make coarse crumbs. Place on top of the batter and gently push down into it.
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Place in preheated oven and bake for approximately 50-60 minutes or until a toothpick inserted into the center comes out clean and topping is lightly brown and crisp.
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Remove from oven & place on a wire rack for at least 10 minutes before cutting.