Step 1- Thaw the Puff Pastry
Take the puff pastry sheets out of the freezer and let them thaw at room temperature for 5 to 10 minutes, until softer and easier to work with.
Step 2- Preheat and Prep
Preheat your oven to 180°C (350°F). Line two baking trays with baking paper or a baking mat.
Step 3- Cut the Squares and Save Scraps
Cut each puff pastry sheet into 4 equal squares, but before folding the edges, trim a few thin strips from the leftover pastry scraps to use for the lattice later. Fold about 1 cm (0.4 inch) of the sides of each square to create an edge, pressing gently so it sticks to the pastry base. Prick the bottom of each square with a fork.
Step 4- Par-Bake the Pastry
Place the pastry squares on the lined trays and bake for 10 to 15 minutes, until slightly puffed and lightly golden. Remove from the oven and set aside.
Step 5- Prep the Apples
While the pastries par-bake, peel, core, and halve each apple, then thinly slice each half.
Step 6- Make the Cinnamon Butter
Mix the melted vegan butter with the brown sugar and cinnamon, and set aside.
Step 7- Assemble the Tarts
Spread a little applesauce onto the center of each par-baked pastry square, then top with sliced apple. Brush the cinnamon-butter mixture over each apple layer.
Step 8- Weave the Lattice
Take your reserved pastry strips and weave them over the top of each apple layer in a simple over-under lattice pattern, pressing the ends gently into the pastry edges to secure them.
Step 9- Add the Wash
Whisk the aquafaba or plant milk in a small bowl and brush it over the lattice and pastry edges for a golden finish.
Step 10- Bake
Bake for 15 to 20 minutes, or until the lattice and edges turn puffed and golden, and the apples soften slightly.
Step 11- Cool and Dust
Let the tarts cool completely before dusting with icing sugar, if using.