Prepare the Puff Pastry Shells:
Thaw puff pastry on the counter, then place one sheet on top of the other. Roll them gently with a rolling pin until the pastry reaches about 12×16 inches. Do not add flour in between the sheets. Doubling them helps create higher tart borders. Cut out circles using a 3-inch cookie cutter, as close together as possible. Transfer to a parchment-lined baking sheet, spacing the circles about 1 inch apart. Press a 2-inch cookie cutter into the center of each circle without cutting all the way through.
Egg Wash & Bake:
Whisk together the egg for the egg wash. Brush the tops of the puff pastry circles and any leftover scraps with the egg wash. Sprinkle with granulated sugar. Bake at 425°F (220°C) for about 15 minutes or until puffed and golden.
Make the Ganache:
While the pastries bake, heat 1/2 cup heavy cream until almost boiling. Pour the hot cream over 1/2 cup chocolate chips and let sit for 2-3 minutes. Stir until smooth. Transfer the ganache to a sandwich-size zip-lock bag for later use.
Prepare Chantilly Cream:
In a mixer bowl, combine 2 cups heavy cream, 2/3 cup granulated sugar, and 1 Tbsp vanilla extract. Whip on high until stiff peaks form. Transfer to a pastry bag or gallon-sized zip-lock bag fitted with a star tip and refrigerate until ready to use.
Assemble Tarts:
Once the puff pastry shells are baked, press down the centers to create a cavity for the filling. Drizzle ganache inside each tart, reserving a small portion for decoration. Pipe Chantilly cream into the center of each tart, using the star tip for a decorative touch.
Garnish:
Place one raspberry and two blueberries on top of each tart. Drizzle reserved ganache in lines across the top of the tarts. Right before serving, dust with powdered sugar for a finishing touch.
Enjoy:
Serve the tarts cold, straight from the fridge. For the leftover puff pastry scraps, bake, top with a dollop of cream, and garnish with a single raspberry.