In a small frying pan over medium heat, melt butter until slightly golden and foaming.
Add apples and brown sugar and increase the heat to high. Cook for 5 minutes or until sugar is dissolved and slightly thickened into a syrup.
Place a sieve over a bowl and drain the apple mixture. Set aside ½ of the mixture, and place the remaining mixture and syrup in a blender and process until smooth.
Place the puree back into the pan and fold through the remaining apple mixture, lemon zest and juice and set aside to cool.
Take out pastry from the freezer and slightly defrost. Using a 12cm round cutter, cut 6 rounds and place 1½ tbsp of the apple mixture in the centre of each round. Fold in half and pinch together the edges. Brush with egg yolk and using a sharp knife, score diagonal lines in the pastry (like a sunset), slicing the pastry ¾ through. Place in the fridge for 1 hour or overnight. Reserve any leftover egg yolk and place in the fridge.
Preheat oven to 210°C (190°C fan-forced) with 30% steam. Line a baking tray with baking paper. Remove pastries from the fridge and brush with remaining egg yolk once more, make two small holes in the long edge of pastry and cook for 25-30 minutes or until golden brown.