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1
Preheat oven to 375 degrees F. Line a 9 x 13-inch baking pan with non-stick foil.
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2
For the cake:.
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3
In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Toss the blueberries in the flour mixture to coat, then remove blueberries with a slotted spoon to a separate bowl. (The flour will help keep the blueberries from sinking and turning the batter gray.) Set both aside.
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4
In a large bowl, cream butter, white sugar, and brown sugar together until fluffy. Add vanilla, egg, and sour cream, beating until smooth.
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5
Add flour mixture to wet ingredients, half at a time, beating until smooth.
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6
Fold in reserved floured blueberries and white chocolate chips. Pour into prepared pan and smooth evenly. (Batter will be stiff.)
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7
For the topping:.
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8
Whisk together flour, white sugar, brown sugar, and cinnamon until combined. Add cold butter and use a pastry blender to mix until coarse crumbs are formed. (Alternatively, place all topping ingredients into the bowl of a food processor fitted with the metal blade and pulse only until coarse crumbs form.) Sprinkle over batter.
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9
Bake 35 to 40 minutes until toothpick inserted in the center comes out clean. Let cake cool to room temperature before cutting to serve.
