Roll out the puff pastry and cut out 36 rounds about 4.5 cm in diameter. Cut them close together so you don't need to re-roll the pastry.
Set 12 of them aside on a baking tray lined with baking parchment.
Stack the remaining 24 rounds in pairs so you have 12 double layers. Cut out the centre of each with a cutter about 3 1/2 cm in diameter.
Brush the bases on the baking tray with egg yolk, place a double ring on each, and brush the tops. You now have 12 mini tartlet cases.
Bake in a preheated conventional oven (top and bottom heat) at 200°C for about 12-14 minutes – or until nicely golden. *Make the mushroom filling while the tartlets bake.
Press the centre down if it has puffed up – then they are ready to be filled.
Creamed mushrooms
Melt the butter in a frying pan and sweat the shallot in it until soft and translucent.
Add the mushrooms and let them brown well over a fairly high heat until all their liquid has evaporated. Season with salt and freshly ground pepper.
Pour in the white wine and let it reduce completely, still over a fairly high heat, then add the cream and the stock cube. Stir regularly and let the mushrooms cook until the cream has reduced well and clings to the mushrooms.The filling should not be saucy like a classic tartlet filling, but denser.
Finally, stir in the chives and season to taste with salt and freshly ground pepper.
Let the mixture cool slightly before transferring it to a piping bag and piping it into the tartlets. Sprinkle with chives and freshly ground pepper before serving, if you like.Note: Make sure your piping bag can withstand hot food. Otherwise, simply fill the tartlets with a small spoon.