Thaw the puff pastry at room temperature for approximately 30–40 minutes, or until pliable but still cool.

Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.

Cut the pastry sheet into 6 roughly equal rectangles, approximately 4.5 x 3.5 inches each.

Lightly score a smaller rectangle inside the top of each pastry without cutting completely through the dough. This creates an area that can be gently pressed down after baking to hold the filling.

Whisk the egg and milk together. Brush the mixture over the pastry and sprinkle evenly with coarse sugar.

Bake for 12–15 minutes, or until the pastries have risen and become crisp and deeply golden. Transfer them to a rack and cool completely.

For the mascarpone filling, whip the cold whipping cream and powdered sugar until soft peaks form. Add vanilla, mascarpone, and optional lemon zest. Whip briefly until thick, smooth, and creamy. Refrigerate until needed.

To prepare the lemon curd, beat the softened butter with granulated sugar and lemon zest until pale and fluffy. Beat in the eggs one at a time.

Mix in the lemon juice and salt on low speed. The mixture may initially appear separated.

Transfer to a heavy-bottomed saucepan and cook over medium-low heat, stirring continuously. Do not allow it to boil. Cook until thick and creamy and the mixture reaches approximately 170°F (77°C).

Transfer the curd to a heat-safe bowl and chill completely in the refrigerator.

To assemble the Lemon Cream Puffs, carefully cut along the previously scored lines and gently press the center of each pastry down.

Pipe or spoon mascarpone cream into each pastry. Add a generous spoonful of lemon curd, followed by mixed berries.

Finish with a light dusting of powdered sugar and serve.