Heat milk, sugar and salt in a sauce pan until it steams.
In a separate bowl with the eggs, slowly pour half of the milk while whisking, return to the pot and cook until slightly thickened, about 3 minutes. Let cool.
Lightly roll the sheets of puff pastry to smooth out the seams. Cut each sheet into 9 squares and gently press into a muffin tin.
Cut the apples in half and remove the core, slice apples very thin, try to keep them stacked together. Take a pinch of apple and fan them out lengthwise into a strip, roll them up and place them in the pastry lined tin. This step is tricky and might take a few tries to get the hang of it. If the apples crack when your curl them then you are cutting them to thick.
Scoop 2 tablespoons of custard into each cup, fill them more if the pastry is tall enough. Sprinkle each with a pinch of cinnamon.
Bake at 350F for 20-25 minutes until the custard is set and the pastry is golden brown. Serve warm or room temperature.