Puff Pastry:

1. Thaw Pastry: Thaw the puff pastry on the counter for 30-40 minutes, or in the microwave. For microwave thawing, wrap in paper towels and microwave for 15 seconds on each side until soft.
2. Cut Pastry: Cut the pastry into 6 rectangles, about 4.5×3.5 inches each.
3. Prepare Pastry: Place rectangles on a baking sheet lined with parchment paper. Score the tops with a knife (don’t cut all the way through) to create a space for the filling.
4. Egg Wash: Whisk the egg and milk together, brush over the tops, and sprinkle with coarse sugar.
5. Bake: Bake at 400°F for 12-15 minutes until golden. If your oven runs hot, bake at 375°F. Let them cool before filling.

Mascarpone Frosting:

1. Whip Cream: Whip the cold heavy cream and powdered sugar until soft peaks form.
2. Mix Ingredients: Add lemon zest (optional), mascarpone cheese, and vanilla extract. Whip until smooth.

Lemon Curd:

1. Cream Butter and Sugar: Beat butter, sugar, and lemon zest in a mixer until light and fluffy.
2. Add Eggs: Add eggs one at a time, mixing well after each.
3. Cook Mixture: Add lemon juice and salt. Stir on low speed until combined.
4. Cook Curd: Transfer mixture to a saucepan. Cook over medium-low heat, stirring constantly, until thickened and creamy (temperature should reach 170°F).
5. Cool: Pour into a bowl and let cool in the fridge.

Assemble:

1. Prepare Pastries: Use a knife to cut along the scored lines on the baked pastry rectangles. Press down the centers to create a space for the filling.
2. Fill Pastries: Pipe or spoon mascarpone frosting into the pastries. Add a dollop of lemon curd on top.
3. Add Toppings: Top with mixed berries and dust with powdered sugar.