Preheat your oven to 350°F
Line 2 cupcake tins with wrappers and spray with non-stick baking spray.
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. Add the dry ingredients and the buttermilk until mix until combined, making sure to scrape down the sides during the process. Remove the bowl from the mixer, and fold in the strawberries. Set the batter aside.
Make the California Walnut Streusel by combining all ingredients 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.