Combine dry dough ingredients (flour, sugar, salt and cinnamon) into a large bowl.
Cut in butter using a pastry blender or squeeze butter pieces with fingertips until pea-sized and coated in flour. Set aside.
In medium bowl, combine wet dough ingredients (cold water, ice cubes and apple cider vinegar) and stir to combine.
Add 2 tablespoons water mixture to flour mixture and discard any remaining solid ice. Mix using pastry blender or hands until water mixture is absorbed into dough. Continue adding water mixture to flour mixture, 2 tablespoons at time, just until dough comes together.
Divide dough into 2 equal portions and pat each dough portion into a rectangle about 5 inches by 6 inches.
Wrap each dough portion tightly in plastic wrap. Refrigerate for 1 hour.
Combine mixed berries, sugar, cornstarch, lemon juice, cardamom, vanilla extract, and salt in bowl.
Once mixed, transfer berry mixture to a saucepan and heat on medium high for 5 minutes or until mixture begins to boil. Make sure all berries are mashed/crushed.
Cook another 7-8 minutes and stir occasionally with spatula. Set aside filling to thicken and cool to room temperature. Chill berry mixture in refrigerator until needed.
Line 2 baking sheets with parchment paper and remove 1 dough portion from the refrigerator. Dust out a surface with flour.
Roll out dough to a rectangle about 10 inches by 15 inches. Cut dough into 12 small rectangles, each about 3 inches by 3 ½ inches. Place onto prepared baking sheet and chill in freezer for 10 minutes. Repeat with second dough portion.
Meanwhile, combine egg wash ingredients into small bowl and wish until combined. Set aside.
Preheat oven to 375°F. Remove 1 tray of small dough rectangles from freezer and brush surface of dough with egg wash.
Reserve and set aside 1 ½ tablespoons berry mixture for use in icing. Scoop 1 tablespoon onto center of each dough rectangle and leave a ½-inch border around edges. Seal tart edges firmly closed using tines of a fork and cut a small X into the top of each tart using a sharp knife. Bush tarts for with egg wash.
Bake 25 minutes or until golden brown.
Press reserved 1 ½ tablespoons berry filling into a bowl. Add sugar, vanilla extract, milk and Great Lakes Mixed Berry Collagen and whisk until combined. Mixture should be thick.
Spread about 2 teaspoons icing onto the surface of each tart.