Streusel Topping: In a large bowl, mix together the flour, sugar, and ground cinnamon. With a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture, until it resembles coarse crumbs. Set aside while you make the cake batter.
Cake Batter: In a separate bowl whisk together the flour with the baking powder and salt.
In the bowl of your electric mixer, or with a hand mixer, beat the butter until smooth. Add the sugar and beat until light and fluffy. Add the egg and vanilla and beat until incorporated. Add the flour mixture (in three additions), alternately with the milk (in two additions), and beat only until combined. Spread the batter onto the bottom of the prepared pan, smoothing the top with the back of a spoon or an offset spatula.
Evenly arrange the blueberries on top of the cake batter and then sprinkle with the streusel topping.
Bake for about 40 - 50 minutes or until a toothpick inserted in the center of the cake comes out clean and the cake just starts to pull away from the sides of the pan. Remove from oven and place on a wire rack to cool slightly.
Serve warm, at room temperature, or even cold.
Makes about 9 servings.
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