Make the Blueberry Sauce: In a small saucepan add blueberry, sugar, and lemon juice. Simmer over medium heat, stirring frequently for 15-20 minutes or until the fruit softens and the sauce thickens. Remove from heat, pour into a bowl, and let cool. The sauce will continue to thicken as it cools.

Prepare the Puff Pastry: Preheat the oven to 425°F (220°C). If using frozen puff pastry, thaw according to package instructions. It should be still cold but manageable. If needed, roll out the puff pastry into a rectangle of about 10 x 15 inches (24 x 36 cm).

Sprinkle lightly with 1 teaspoon of sugar. Cut into six squares of 5-inch (12 cm) sides. Place on a baking sheet lined with parchment paper. Grab one corner and fold each square into a triangle. Press and seal lightly the edges to ensure the triangle doesn't pop open too much in the oven.

In a small bowl, beat the egg with the milk, and brush the top of the pastries with the egg wash. Sprinkle with the remaining sugar. Bake for about 20-22 minutes or until golden brown. Start checking around 18 minutes to make sure the top doesn't burn. Let cool completely.

While the pastries bake, prepare the Mascarpone Whipped Cream: In a large mixing bowl, beat cold heavy cream with a hand mixer until stiff peaks form. In a separate medium bowl, beat the mascarpone cheese with powdered sugar, and vanilla extract until smooth.

Gently mix or fold the mascarpone cream into the whipped cream until fully combined. Adjust sweetness to taste. Place into a piping bag and store in the refrigerator until ready to serve.

When ready to serve assemble the Pastries: Use a sharp knife to slice each cooled pastry in half (but not all the way through, just enough to add the filling) . Spread a generous amount of blueberry sauce inside each pastry (top and bottom). Pipe the mascarpone whipped cream to fill the pastries. Close the pastries and dust the tops with powdered sugar. Serve immediately for the best texture and flavor

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