On a lightly floured surface, roll puff pastry sheet into a 5 x 12 inch rectangle. With the longer side facing you, roll tightly into a log. Divide the log into 12 equal pieces. With the cut side facing up, press each piece of dough into an ungreased muffin tin so the sides cover well. Flatten with a rolling pin if necessary. Once all cups are lined with dough, chill in the refrigerator.
Preheat oven to 500F/260C for 1 hour if possible. For the custard, whisk together milk and flour in a medium saucepan. Heat on medium high, whisking constantly. It will come to a boil and begin to thicken. Lower heat and cook for 1 minute. Remove from heat.
Whisk egg yolks in a small bowl and add about ¼ cup of the hot milk mixture slowly. Whisk quickly so it doesn't turn to scrambled eggs. Add the egg mixture back into the pot and return to heat. Cook another 30 seconds until it thickens slightly. Remove from heat and add vanilla.
In a small pot combine sugar, water and salt. Heat on high without stirring until it comes to a boil. Reduce heat to medium low and cook for 3 minutes. Remove from heat and add to the custard, whisking until smooth. The mixture will be thin. Set aside to cool until lukewarm.
Once the oven is blazing hot, remove chilled puff pastry cups from the fridge and fill with the cooled custard. Bake immediately for 15-20 minutes or until the tops are blistered and pastry is golden. Cool 5 minutes before removing from muffin tin. Serve warm or cold.