01 - Preheat the oven to 400°F. Line two baking sheets with parchment paper.
02 - Place the puff pastry sheets onto the prepared baking sheets. Prick the surface thoroughly with a fork to prevent excessive rising during baking. Place a second sheet of parchment on top, then set another baking sheet over it to weigh down the pastry as it bakes.
03 - Bake for 15 to 18 minutes until the pastry turns a deep golden color and feels crisp to the touch. Remove the top baking sheet and parchment layer, then allow the pastry to cool completely on the pan.
04 - While the pastry cools, combine the ricotta cheese, powdered sugar, lemon zest, lemon juice, and vanilla extract in a medium mixing bowl. Whisk until the mixture is completely smooth and free of lumps.
05 - In a separate bowl, whip the heavy cream until soft peaks hold their shape. Gently fold the whipped cream into the lemon ricotta mixture in three additions, preserving as much volume as possible. Refrigerate until ready to assemble.
06 - Once the pastry has cooled completely, carefully trim any uneven edges with a sharp knife. Cut each sheet into 3 equal rectangles, yielding 6 pastry layers total.
07 - Place one pastry rectangle on a serving plate. Spread one-third of the lemon ricotta filling evenly over the surface. Top with a second pastry rectangle and another third of the filling. Finish with the final pastry rectangle on top. Repeat the process to build a second mille feuille with the remaining layers and filling.
08 - Dust the top layer lightly with powdered sugar using a fine-mesh sieve. Garnish with lemon zest curls if desired. Refrigerate for at least 30 minutes before slicing to allow the layers to set. Serve chilled.