2 hours before you begin, take your filo dough out of the freezer and let it defrost. You can also let it defrost overnight in the refrigerator, if you prefer.
Preheat your oven to 325F. Cut the cored, peeled apples into very thin slices.
Place the slices in a mixing bowl along with the brown sugar, vanilla, ½ tsp of cinnamon, and salt. Toss all ingredients with a spoon till the apple slices are evenly coated. Let them macerate for a few minutes so the apples become juicy and the sugar begins to dissolve.
Lightly dampen a kitchen towel (don’t soak it!), and keep it with you as you work with the filo dough. Unroll your dough. Filo sheets come in different sizes, so you may need to cut your sheets to size get the size you need. For each boureka, you want to create a double layer of filo dough that is about 4 inches wide and 14 inches long.
Folding a smaller filo sheet (8x14) in half lengthwise is the easiest way to form this double layer.
If your filo sheets are larger, use scissors to cut the sheet in half lengthwise to make two rectangular pieces.
Then, fold each piece in half lengthwise to create the double layer of filo you need.
Keep the unused filo sheets under the damp towel as you work to keep them from drying out. Filo is very delicate and prone to ripping, so if you’ve never worked with it before, be patient… rips will happen. It takes some time to get a feel for it.
Use a pastry brush to pain a thin layer of melted butter onto the surface of the double layer filo rectangle.
Place a few slices of the apple filling in the lower left corner of the filo rectangle, following the left edge of the filo and fanning downward in a triangular shape.
Fold the lower left corner of the rectangle up and over the apple filling to form a triangle of dough.
Take the upper left and corner and fold it up and over, maintaining the triangular shape.
Continue folding the filo in the same way you would fold a flag, till the entire strip is used up and a neat triangle pastry is formed.
Place the folded boureka on an ungreased baking sheet.
Repeat process until all the filling is used up. You should end up with around 14-15 bourekas. You may need two baking sheets to fit all of the bourekas.
Brush each boureka with another thin layer of melted butter. Don’t brush it on too heavy, or the bourekas will become very greasy. Just a light layer will do it.
In a small bowl, mix together 2 tbsp of sugar and ½ tsp of cinnamon. Sprinkle the cinnamon sugar over the top of the buttered bourekas.
Place the bourekas on the middle rack of your preheated oven. Let them bake for 20-25 minutes till filo turns golden brown and the filling begins to bubble out in places. Serve warm.