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Preheat oven to 350 degrees Fahrenheit.
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Grease and flour a 9”x11” cake pan and line bottom with parchment paper.
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In large bowl, mix together 2 cups of flour and the baking powder.
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In separate bowl, mix together until creamy, eggs, oil and 1 cup of sugar.
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Pour over flour mixture and blend well.
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In large bowl, mix together rhubarb and remaining flour and white sugar. (Do not make this mixture ahead of time as the rhubarb will become very tallowy.).
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Pour batter over the rhubarb mixture and stir until rhubarb is covered with batter and pour into prepared pan.
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Mix brown sugar, oats and cinnamon together and sprinkle over top.
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Cook for 30 minutes or until center springs back.
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Let cool on wire rack ½ an hour before removing from pan.