Blueberry Crumb Cake with Walnut Streusel

Blueberry Crumb Cake:
2 cups all purpose flour
1 tsp baking powder
1 tsp baking soda
1 tsp salt
8 Tbsp butter, at room temperature
1 1/4 cups white sugar
2 Tbsp fresh lemon zest
1 large egg
1 egg yolk
1 tsp vanilla
1 cup buttermilk
1 1/2 cup fresh blueberries

Walnut Streusel:
1 cup all purpose flour
1 cup light brown sugar
2/3 cup chopped walnuts
8 Tbsp butter, melted
2 Tbsp fresh lemon zest
1 tsp sea salt

Preheat oven to 350.  Spray 9x9 pan with nonstick cooking spray.  In small bowl, sift together the flour, baking powder, baking soda, and salt and set aside.  In a large mixer, combine butter and sugar until pale yellow, add zest, egg, egg yolk and vanilla.  Beat well for an additional 3 minutes.  Alternate adding the flour mixture and the buttermilk until all is combined.  Fold in blueberries.  Set the batter aside.  

Make walnut streusel by combining all ingredients in a small bowl.  Using your fingers, mix it around until its crumbly and set aside.  

Fold half of the streusel mixture into the batter and stir to combine.  Pour batter into prepared pan and top with remaining streusel mixture.  Bake for 55-65 minutes or until a toothpick inserted in center comes out clean.