Start by washing and drying your mixed berries—strawberries, raspberries, and blueberries—to remove any moisture. Chop the strawberries into smaller pieces while keeping the raspberries whole for delightful texture. To ensure your Spring Berry Cookies aren't soggy, blot the berries gently with a paper towel before setting them aside.

In a medium mixing bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon baking powder, 1 tablespoon cornstarch, and a pinch of salt. This step is crucial for aerating the flour and ensuring the leavening agents are evenly distributed. Set this mixture aside.

Using a hand mixer or stand mixer, beat together ½ cup of unsalted butter and 4 ounces of cream cheese on medium speed until the mixture is smooth and creamy. Gradually add ½ cup of granulated sugar and ½ cup of light brown sugar, mixing until fluffy and well combined.

With the mixer on low speed, incorporate 1 large egg, 1 teaspoon of vanilla extract, and the zest of 1 lemon into the creamed mixture. Beat until everything is just combined.

Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Next, carefully add in your prepared berries, folding them in gently to distribute them throughout the dough while retaining their shape and juices.

Cover the bowl with plastic wrap or a lid and chill the dough in the refrigerator for about 20-30 minutes.

Preheat your oven to 350°F (175°C) while the dough is chilling. Line your baking sheets with parchment paper. Use a cookie scoop or spoon to place rounded tablespoons of dough onto the sheets, leaving some space between each scoop. Bake for 10-13 minutes.

After removing the cookies from the oven, allow them to sit on the baking sheet for about 5 minutes before transferring to a wire rack. Once moved, let them cool completely.