Prepare the blueberry filling:

In a small saucepan, combine the wild blueberries, lemon juice, granulated sugar, cornstarch, and cinnamon. Cook over medium heat, stirring frequently, until the mixture thickens and the blueberries break down slightly (about 5-7 minutes). Remove from heat and set aside to cool.

Make the streusel topping:

In a medium bowl, combine the flour, granulated sugar, brown sugar, and cinnamon. Add the cold cubed butter and use a pastry cutter or your fingers to blend the ingredients until the mixture forms coarse crumbs. Set aside.

Prepare the puff pastry:

Preheat your oven to 375°F (190°C). Roll out the puff pastry sheet on a lightly floured surface. Cut the pastry into 6-8 squares or rectangles, depending on your preferred size for the danishes.

Assemble the danishes:

Place the pastry squares or rectangles on a baking sheet lined with parchment paper. Using a fork, gently score a border around the edge of each pastry, about 1/2 inch in from the edge (this helps create a nice puff around the filling).

Spoon a generous amount of the blueberry filling into the center of each pastry.

Add the streusel topping:

Sprinkle the cinnamon streusel topping evenly over the blueberry filling, pressing it lightly into the filling.

Apply the egg wash:

Lightly beat the egg and brush it over the edges of the puff pastry to give it a golden finish when baked.

Bake the danishes:

Place the baking sheet in the preheated oven and bake for 15-20 minutes, or until the puff pastry is golden and puffed up, and the streusel is crispy.

Prepare the glaze:

While the danishes are baking, whisk together the powdered sugar, milk or cream, and vanilla extract in a small bowl until smooth.

Finish the danishes:

Once the danishes are done baking, remove them from the oven and let them cool slightly. Drizzle the glaze over the danishes while they are still warm.

Serve:

Serve the Wild Blueberry Coffee Cake Danishes warm or at room temperature. Enjoy them for breakfast, brunch, or as a sweet treat with coffee or tea.