How to Make It
In a food processor, pulse the flour with the butter and salt until the mixture resembles coarse meal. Pour the beaten egg evenly over the mixture and pulse several times, just until evenly moistened. Transfer the pastry to a work surface and gather it into a ball. Flatten the pastry into a disk, wrap in plastic and refrigerate for at least 30 minutes.
Preheat the oven to 350°. On a lightly floured surface, roll out the pastry to a 14-inch round a scant 1/8 inch thick. Using a 2 1/4-inch round cookie cutter, stamp out as many rounds as possible. Reroll the scraps and stamp out more rounds.
Line the cups of three 12-cup mini-muffin pans with the pastry rounds, pressing the pastry into the molds. Refrigerate until firm.
Cut out thirty-six 2-inch squares of foil and line the pastry with them. Bake for 15 minutes, or until the shells are dry. Remove the foil and let cool.
In a heatproof medium bowl, soak the porcini in the boiling water until softened, about 20 minutes. Lift the porcini out of the soaking liquid and pat dry, then finely chop them.
In a medium skillet, melt the butter in the olive oil. When the foam subsides, add the porcini and cook over moderately high heat, stirring, until golden, about 3 minutes. Add the garlic and cook until fragrant, about 1 minute. Season with salt and pepper and stir in the parsley. Transfer the mushroom mixture to a bowl and let cool, then whisk in the cream, whole egg, egg yolk and Fontina.
Spoon the mushroom custard into the pastry shells and sprinkle with the chives. Bake at 350° for about 15 minutes, or until set. Let the pomponnettes cool slightly before serving.