Preheat oven to 350°. Cover baking sheets with parchment paper or silicone baking sheets.
In mixing bowl mix together the flour, baking powder, baking soda, salt and cornstarch. Set aside.
Melt butter in microwave and allow to cool until no longer hot. Can place in the refrigerator to chill.
Add sugars to butter and whip until smooth. Add lemon zest and lemon juice and whip together.Beat in egg and egg yolk.
Add in dry ingredients and mix until just incorporated.Stir in blueberries and *1 cup white chocolate chips.**If using frozen berries, keep frozen until ready to mix in.
Use a heaping large cookie scoop for each cookie. (About 3-4 Tbsp).Place about 2 inches apart, forming into balls and adding in additional blueberries as needed. Use your hand to gently smash each cookie.
Bake for 14-16 minutes until cookies are browning around the edges.Cool for 5 to 10 minutes before removing to cooling rack.
Melt remaining white chocolate chips in the microwave in 20 second bursts until you can stir smooth with a spoon.Use spoon to drizzle cookies.
Keyword baking, bar cookies, blueberries, lemon cookies, white chocolate