Preparation

• Make the blueberry mixture. Add the blueberries and sugar into a small saucepan. Cook over medium heat while stirring frequently and try to leave most of the berries intact. Continue stirring for 5-7 minutes until the mixture has reduced by about half. Stir in lemon juice. Take about 1 tablespoon of the blueberry juices and stir them into the corn starch in a little bowl to make a slurry then pour it into the saucepan. Stir it in and boil for 10 seconds then transfer to a clean bowl and let cool. Refrigerate until thoroughly chilled.
• Make the cookie dough. Combine the soft butter, both sugars, lemon zest, and vanilla extract in a large bowl and mix using an electric handheld mixer (or mix it in the bowl of a stand mixer fitted with the paddle attachment) on medium speed for 1-2 minutes until it is pale and fluffy. You can also mix by hand using the creaming method with a wide rubber spatula. Mix in the egg until well combined.
• Combine flour, baking soda, baking powder, and salt in a medium bowl and whisk to blend evenly. Add the dry ingredients to the butter mixture and mix gently on low speed or fold it in by hand until mostly incorporated, then fold in the white chocolate chunks until evenly distributed.
• Swirl the dough. Divide the dough in half and place one half into another clean bowl. Spread the dough out along the bottom of the bowl so there is more surface area for the blueberry mixture. Dollop a total of ¼ cup of the chilled blueberry mixture in small spoonfuls into each bowl randomly over the dough. Use a butter knife to cut it through the dough until it is barely distributed. Place the bowl with the dough in the fridge to chill while you preheat the oven.
• Preheat your oven to 350°F. Line two large cookie sheets with parchment paper.
• Use a 1.35-oz cookie scoop to drop mounds of dough onto prepared baking sheets spacing them about 3 inches apart. Place extra white chocolate chunks on top if desired.
• Bake for 14-16 minutes until browned around the edges and on the bottoms, and lightly golden on top. Let them cool for 2 minutes on the baking sheets and then transfer to a wire rack to finish cooling.