Preheat your oven to 350°F. Line a 12-cup muffin tin with cupcake liners.

In a large mixing bowl, cream together the softened butter and sugar until light and fluffy, about 3-4 minutes. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Mix until just combined.

Divide the batter evenly into two bowls. In one bowl, gently fold in the blueberries. In the other bowl, fold in the diced strawberries.

Spoon the blueberry batter into the bottom half of each cupcake liner, filling them about ⅓ full. Top with the strawberry batter, filling each liner about ⅔ full.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

Once cooled, frost each cupcake with vanilla frosting and decorate with red and blue sprinkles.