Thaw the puff pastry if frozen and unfold it on a lightly floured pastry board. Cut into 4x4inch squares if it is a thin long sheet. Otherwise if it is a big thick square, cut it into smaller squares and roll into 4x4 inch squares.
For the apple filling, peel, core and slice the apples. Add a blob of butter to the sauce pan followed by the apple slices, brown sugar, cinnamon and vanilla essence. Combine well and cook till the apples caramelize and are no longer runny. Remove to cool and set aside.
Place one or two teaspoons of filling in the centre of each pastry square, brush the edges with egg white and fold into a triangle. Then press the edges using your fingers.
You could be a bit creative to make a design on the edges with the fork or cookie cutter. This not only makes them look pretty but helps to seal them well. You could also make square shaped hand pies by using 2 pastry squares one on top of the other with the filling inside.
Prick with the fork in the centre to let the air out before putting them in the oven. This prevents the pastry from bubbling while baking. Glaze them with egg white and place them on a cookie sheet lined with parchment paper. It is better to keep them chilled in the fridge for sometime before placing them in the oven.
Bake in the oven preheated at 375 degrees fahrenheit for 30 - 35 minutes till golden brown.
Cool them on a wire rack and enjoy with tea or vanilla ice cream.