Preheat oven to 425 F. Lightly grease 12 muffin cups with melted butter.

Using a biscuit cutter, cut puff pastry into twelve 3" rounds. With your hands, gently stretch the rounds out until they are approximately 4" in diameter and place one round in each muffin cup. Press pastry into bottom and sides of the cups; they should be lined to within approximately 1/4" of the top.

Whisk together sugar and flour in a 3-quart heavy saucepan, then whisk in remaining custard ingredients and cook over moderate heat, stirring constantly, until bubbles just start to appear on the surface, about 10 minutes. Transfer custard to a bowl and cool, whisking occasionally, until just warm, about 15 minutes.

Fill pastry cups with custard (about 2 - 3T each) and bake in upper third of oven until pastry is deep golden, about 20 minutes. Custard should be starting to brown and there may be the odd burned bit on the edges.

Cool tarts slightly (in the muffin tin) on a rack, about 10 minutes. Remove from tin, then sift first icing sugar, then cinnamon, over tarts. Serve warm or at room temperature.