Heat oven to 350Grease and flour a 10” springform pan and line bottom with parchment.
Make streusel: Mix dry ingredients together, cut in butter until fine and crumbly. Set aside.
Combine flour, baking powder, and salt and set aside. Wash and thoroughly dry blueberries.
Cream butter and sugar with an electric mixer on high until fluffy. Add egg, zest, and vanilla. Continue blending on med-low until pale and creamy.
Keep mixing on low. Alternate adding flour mixture and milk to mixture, beginning and ending with flour mixture.
Fold in blueberries.
Scrape into prepared pan. Spread to edges and flatten. Sprinkle topping evenly over cake.
Bake 60-70 minutes or until cake tester comes out clean.
Remove from oven and let cool 10 minutes in pan. Remove from pan and allow to cool at least 15 more minutes before serving.
Stays well overnight or longer. Flavors bend nicely with age.