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.

To make the topping: Mix the sugar, flour, cinnamon, and salt in a small bowl. Use a microplane zester to zest in 1/2 teaspoon lemon zest into the bowl and stir to distribute.  Cut or rub in the butter with the side of a fork, two knives or your finger tips until it reaches a crumbly state.  Set aside.

Blend the flour, baking powder, and salt together in a medium-sized mixing bowl.

In a separate bowl, Add sugar, butter, egg, and vanilla and then zest in all the remaining peel from the lemon.  

Alternately add the milk and the flour mixture to the sugar/butter mixture, ending with flour. Add blueberries. Stir only enough to blend.

Pour the batter into the prepared pan. Sprinkle the topping over the batter.

Bake the cake for 40 to 45 minutes, or until a toothpick or knife inserted into the center comes out clean.

Remove the cake from the oven, and set it on a rack to cool for 10 minutes.

While cake is cooling, place 1/2 cup powdered sugar in a small bowl and whisk in enough lemon juice to make a glaze.  Drizzle over slightly warm or cool cake. 

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.

Yield: about 12 to 16 servings.