Step 1: Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper or silicone mats.
Step 2: In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
Step 3: In a large bowl, using an electric mixer, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 2-3 minutes).
Step 4: Beat in the large egg, then the egg yolk, lemon zest, and vanilla extract until well combined, scraping down the sides of the bowl as needed.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.
Step 6: Gently fold in the white chocolate chips until evenly distributed.
Step 7: Carefully fold in the fresh mixed berries. It's important to do this gently to prevent crushing the berries too much, which can cause excessive moisture in the dough.
Step 8: Drop rounded tablespoons of dough onto the prepared baking sheets, leaving about 2 inches between each cookie.
Step 9: Bake for 12-15 minutes, or until the edges are golden brown and the centers are just set. The cookies may still look slightly soft in the middle; they will firm up as they cool.
Step 10: Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Enjoy these wonderful summer cookie recipes!