Sift flour, baking powder, soda, and salt in a medium basin. Break frozen blackberries with your hands. It must be 1 cup packed. Set aside.
1
Beat melted butter (cooled to room temperature), sugar, and brown sugar on high speed for 2 minutes in a mixer.Mix lemon zest, juice, egg, and yolk over medium until mixed.
2
Stir in the dry ingredients on low speed until mixed. Disconnect the mixer and add blackberries. Mix dough with a rubber spatula.Heat the oven to 350°F. Use parchment on two cookie sheets. Rest dough for 10 minutes. This aids spreading.
3
Use a 2-TBSP big cookie scoop. Per sheet, scoop 6 cookie dough balls.Baking one sheet at once. Bake until edges are gently browned, 14-16 minutes.
4
The cookies take longer to bake due of the blackberries.Transfer the cookie to a cooling rack after 5 minutes on the heated pan.
5