01 - Preheat the oven to 400°F. Line two baking sheets with parchment paper.
02 - Lay the puff pastry sheets flat on the prepared baking sheets. Prick the surface evenly with a fork to prevent excessive rising during baking. Top each sheet with another piece of parchment paper and place a second baking sheet on top to weigh them down.
03 - Bake for 15 to 18 minutes until the pastry turns a deep golden color and feels crisp. Remove the top baking sheet and parchment layer, then allow the pastry to cool completely on the pan.
04 - While the pastry cools, place the ricotta cheese, powdered sugar, lemon zest, lemon juice, and vanilla extract in a medium mixing bowl. Whisk until the mixture is smooth and free of lumps.
05 - In a separate chilled bowl, whip the heavy cream with a whisk or electric mixer until soft peaks hold their shape. Gently fold the whipped cream into the lemon ricotta mixture in three additions, preserving as much air as possible. Refrigerate until ready to assemble.
06 - Once the pastry sheets are completely cool, use a sharp knife to trim away any uneven edges. Cut each sheet into 3 equal rectangles, yielding 6 total pieces.
07 - Place one pastry rectangle on a serving plate. Spread one-third of the lemon ricotta filling evenly over the surface using an offset spatula. Set a second pastry rectangle on top and spread another third of the filling. Finish with the third pastry rectangle, leaving the top bare.
08 - Dust the top pastry layer lightly with powdered sugar through a fine-mesh sieve. Add lemon zest curls if desired. Refrigerate the assembled mille feuille for at least 30 minutes before slicing to allow the layers to set. Serve chilled.