Preheat your oven to 350°F
Line 2 cupcake tins with wrappers and spray with non-stick baking spray.
In small bowl, sift together the flour, baking powder, baking soda, and salt and set aside.
In a large mixer combine the butter and sugar until pale yellow, add the zest, egg, egg yolk, and vanilla and beat well for an additional 2-3 minutes making sure to scrape down the sides. Alternate adding the flour mixture, and the buttermilk until all is combined, making sure to scrape down the sides during the process. Remove the bowl from the mixer, and fold in the blueberries. Set the batter aside.
Make the California Walnut Streusel by combining all ingredient in small bowl. Using your fingers, mix it around until it’s crumbly and set aside.
Fold half of the streusel mixture into the batter and stir to combine. Spoon 1/4 cup of the batter into each cupcake mold. Sprinkle the top with a few teaspoons of the streusel. Bake for about 30-35 minutes, or until the cake is light and fluffy and a toothpick can be inserted into the center and is clean when removed. Remove from oven and let cool before serving.
Garnish with additional chopped California walnuts.