In a large bowl, combine the whole wheat flour, coconut sugar, and salt. Add the butter and use a pastry cutter to work it into the flour mixture, until the butter becomes about the size of peas. Work quickly to keep the dough cold.
Add the ice water 1 tablespoon at a time and mix thoroughly until dough becomes crumbly and just hydrated enough to hold together.
Pour the mixture onto a floured surface and form the dough into a disc with your hands. Cover and chill for 30 minutes.
Meanwhile, make the jam: Add the strawberries, blueberries, and coconut sugar to a small pot over low heat. Simmer for about 5 minutes, then mash the blueberries with a fork.
Let simmer for another 5-10 minutes, until mixture starts to thicken to a jam-like consistency. Remove from the heat and let cool.
Preheat the oven to 350˚F (180˚C). Line a baking sheet with parchment paper.
Roll out the dough on a floured surface to about ¼- to ½-inch (6 mm to 13 mm) thick.
Using a pizza cutter and a ruler, cut out 10 even rectangles, each about 3x4½ inches (7x12 cm) . You may need to re-roll any excess dough to get all 10 rectangles.
Place 5 of the rectangles on the prepared baking sheet and fill with a heaping tablespoon of the jam. Spread evenly in the center of the rectangles, stopping about 1 inch (2 cm) from the edges, to prevent leakage.
Top with the remaining rectangles and gently press down the edges with a fork to seal the pastries.
Use a pastry brush to gently brush the tops of each pastry with egg wash.
Bake for 15-20 minutes, or until golden brown. Let cool completely.
If desired, make the glaze by whisking together the powdered sugar, vanilla, and milk in a medium bowl until a thick glaze forms.
Use a spoon or rubber spatula to spread the glaze over the pastry tarts.
Serve immediately or store in the fridge for up to 3 days.