Blueberry Coffee Cake

For the streusel topping
6 tablespoons packed light brown sugar
1/2 cup all-purpose flour, spooned into measuring cup and leveled off
1 teaspoon ground cinnamon
1/4 teaspoon salt
4 tablespoons unsalted butter, cold, cut into 1/2 inch chunks

For the cake
2 cups all-purpose flour, spooned into measuring cup and leveled off
2 teaspoons baking powder
1/2 teaspoon salt
1 stick (1/2 cup) unsalted butter, softened
3/4 cup granulated sugar
2 large eggs
1 1/2 teaspoons vanilla extract
1 teaspoon packed lemon zest, from 1 lemon
1/2 cup milk
2 cups fresh blueberries

Instructions
Preheat the oven to 375 and set the oven rack to the middle position.  Grease a 9 inch square pan with nonstick cooking spray.

Make the streusel topping: combine the brown sugar, flour, cinnamon, and salt in a small bowl.  Use the pastry blender, mix until no lumps of brown sugar remain.  Rub in the butter with the pastry blender until it reaches a crumbly state.  Refrigerate until you are ready to use.

In a medium bowl, whisk together the flour, baking powder and salt.  Set aside.

In the bowl of the electric mixer, beat the butter and granulated sugar until creamy, about 2 minutes.  Add the eggs, one at a time, scraping down the sides of the bowl and beating well after each addition.  Beat in the vanilla extract and lemon zest.

Gradually add the flour mixture, alternating with the milk, beating on low speed to combine.  Add the berries to the batter and fold gently with a spatula until evenly distributed.  Do not over mix.

Transfer the batter to the prepared pan and spread evenly.  Sprinkle the streusel topping every over the batter.

Bake for 40-45 minutes, until golden brown around the edges and a cake tester comes out clean.  Let cool in the pan on a rack for about 20 minutes, then serve right from the pan.

The cake is best served the day it is made.  Leftovers will keep for a few days wrapped in foil and stored at room temperature.