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Preheat the oven to 350°F. Lightly grease an 8" round pan with at least 2" tall sides (or a 9" round pan).
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To prepare the fruit: Toss the fruit with the brown sugar and cinnamon. Transfer the fruit to the prepared pan, spreading it in an even layer.
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To prepare the cake batter: Whisk the baking mix and sugar together.
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In a separate bowl, whisk together the oil, eggs, milk or buttermilk, and vanilla.
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Add the wet ingredients to the dry ingredients, stirring just until blended. Spread the batter into the pan, evenly covering the fruit.
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To make the streusel: Mix the baking mix, oats, brown sugar, and cinnamon together until evenly blended. Work in the butter until the streusel is evenly crumbly.
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Sprinkle the streusel uniformly over the cake batter in the pan, pressing it gently into the batter.
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Bake the cake for 35 to 40 minutes, until it's firm on top and the edges of the crust are brown.
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Remove the cake from the oven, run a spatula around the edge, wait 3 minutes, then turn the pan over onto a serving plate. Wait another 30 seconds, then lift the pan off. If anything sticks to the pan, just scrape it out and spoon it back onto the cake.
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Serve the cake warm or at room temperature, dusted with confectioners' sugar, if desired.
