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Preheat the oven to 375°F. Lightly grease an 8" square, 9" square, or 9" round cake pan. If you use an 8" square pan or round pan, make sure it's at least 2" deep.
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To make the streusel topping: In a small bowl, mix the sugar, flour, cinnamon, and salt. Cut or rub in the butter with the side of a fork, two knives or your fingertips until it reaches a crumbly state. Set aside.
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To make the cake: In a medium-sized mixing bowl, whisk together the flour, baking powder, and salt.
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In a separate bowl or the bowl of your stand mixer, beat together the sugar, butter, egg, and vanilla.
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Alternately add the milk and the flour mixture to the sugar/butter mixture, ending with flour. Add blueberries. Stir only enough to blend.
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Pour the batter into the prepared pan. Sprinkle the streusel topping over the batter.
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Bake the cake for 40 to 45 minutes, or until a toothpick or knife inserted into the center comes out clean.
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Remove the cake from the oven, and set it on a rack to cool for 10 minutes. Serve right from the pan. Or, to transfer to a serving plate, Loosen the sides with a knife or spatula. Holding the pan in your left hand, gently tip the cake out onto your right hand, remove the pan, and gently right the cake onto a serving dish.