Thaw the puff pastry. To thaw the puff pastry you can leave it on the counter for 30 to 40 minutes. Thawing time will depend on how hot or cold the kitchen is.
Or you can thaw the pastry in the microwave. Remove the pastry from the box, and remove from the plastic of paper wrapping. Wrap the pastry in a couple of sheets of paper towel. Place it in the microwave for 15 seconds. Turn the pastry over, microwave for another 15 seconds. The pastry should be soft and open easily. If the pastry is still not opening easily, microwave just for another few seconds.
Cut the pastry into 6 rectangles, measuring about 4.5×3.5″, it’s ok if they are a bit bigger or smaller.
Place the rectangles on a baking sheet lined with parchment or silicone.
Score the top of each rectangle with a knife, but don’t cut all the way through. This is done so you can push down on it later and pipe the filling inside.
Whisk the egg and milk together. Brush egg/milk mixture on top and sprinkle coarse sugar.
Bake at 400 Fahrenheit for 12 to 15 minutes until puffed up and golden. You can bake at a lower temperature, around 375 Fahrenheit if your oven runs hot, or if it's convection. Baking time may vary from one oven to the other.
Let it cool down before filling.
Mascarpone Frosting
Start by whipping the heavy cream and powdered sugar until soft peaks form. Make sure the heavy cream is super cold.
Add the zest of one lemon, it's optional but I love it.
Add the mascarpone cheese and vanilla.
Whip again until smooth.
Lemon Curd
Beat the butter at medium speed in the bowl of an electric mixer with the sugar and lemon zest until the mixture is light in color and fluffy.
Add the eggs, one at a time, waiting until one egg has been completely incorporated before adding the next one.
Add the salt and lemon juice and stir on low speed. The mixture may look curdled and separated at this point. That's ok.
Pour the mixture in a small sauce pan with a heavy thick bottom, and cook over medium-low heat, while stirring nonstop.
You are looking for a very thick and creamy curd. Don’t stop stirring, don’t look away. And don’t let the curd actually come to a boil.
The temperature of the curd should be reach 170F.
When that happens, remove from the heat and pour into a heat proof bowl.
Let it cool in the fridge.
The shelf life of lemon curd will be up to 1 week in the fridge, and up to 3 months in the freezer, if well packaged in a closed, air tight container.
To assemble
To assemble the pastries, use a knife to cut around where the lines you scored previously were. Then push down in the center. Pipe or spoon the mascarpone frosting in the pastries. Top with lemon curd. Then top with berries and dust with powdered sugar.
Storage
Store the pastries in the fridge for up to 2 days, but they are best served immediately. They won't get soggy, but the dough might get a bit hard from being in the fridge, so let it come to room temperature before serving.