Preheat your oven to 400°F and line a baking sheet with parchment paper. While the oven heats, measure out all your dry ingredients and have them ready to go. This ensures you won't scramble mid-recipe and keeps the dough from over-mixing.

Whisk together the all-purpose flour, cornstarch, baking powder, and salt in a medium bowl. The cornstarch adds a delicate, tender crumb to these cookies—it's a small detail that makes a big difference in texture. Set this mixture aside.

Microwave the frozen blueberries for 1 minute until they begin to soften and release their juices. Using frozen blueberries prevents the cookies from becoming too wet since fresh ones would add extra moisture. Mash the berries with a fork until they form a thick jam-like consistency with some small berry pieces remaining for texture.

In a large bowl, beat the softened butter and granulated sugar together for 2-3 minutes until the mixture is light, fluffy, and pale in color. This creaming process incorporates air into the dough, which helps the cookies rise and creates a tender crumb. Don't skip this step—it's crucial for the right texture.

Add the blueberry jam from Step 3 to the creamed butter-sugar mixture and fold in gently. Then stir in the vanilla extract and fresh lemon zest, mixing until everything is evenly distributed. The lemon zest brightens the blueberry flavor without making the cookies taste citrusy—I find just this amount creates the perfect balance.

Add the dry ingredient mixture from Step 2 to the wet ingredients and fold together gently with a spatula until just combined—do not overmix. Once the flour is mostly incorporated, fold in the white chocolate chips. Overmixing develops gluten and makes cookies tough, so stop as soon as you no longer see streaks of flour.

Scoop the dough onto the prepared baking sheet in roughly 1-tablespoon-sized portions, spacing them about 2 inches apart. Gently flatten each mound with the bottom of a glass or your palm to about 1/4-inch thickness. Bake for 10-12 minutes until the edges are lightly golden and the centers still look slightly underbaked—they'll continue cooking on the hot pan after removal.

Remove the baking sheet from the oven and let the cookies cool on the pan for 5 minutes—this allows them to set just enough without becoming too firm. Then transfer them to a cooling rack to cool completely. This prevents the bottoms from over-baking and keeps the cookies tender inside with slightly crispy edges.