Cut the cold butter into small cubes. Add butter, flour and confectioner's powdered sugar to a large bowl. Mix together using a pastry cutter, fork or your fingertips until butter is fine and crumbly. It should resemble the size of peas.
Add half of the lemon juice to the flour mixture and mix until it comes together into a pastry dough. Only add more lemon juce if necessary. Use hands in the last part of mixing to form the dough into a ball. Do not over knead.
Roll the dough onto baking parchment paper. Place another piece of parchment on top and roll out even further to create a 12" circle (it's ok if it's not perfect).Place the dough in the fridge for 10 minutes.
While dough is chilling, gently mix together fresh berries, 3 Tbsp confectioners sugar, lemon zest and cinnamon (raspberries and blackberries are more delicate and need to be mixed gently).
Remove top layer of parchment paper from pastry dough. Place dough with bottom parchment on to a baking sheet. Brush the top with beaten egg whites. Pile the berry mixture in the middle.
Fold the edges of dough over to the middle. Gently pressing each corner to secure it.
Brush the top dough with more egg white. Bake for 30-35 minutes or until edges are golden brown. Dust the top with confectioners powdered sugar. Slice into pieces, serve and enjoy!