Preheat the oven to 450ºF.

After trimming bread of its crusts, use a knife to cut a 1 1/4-inch hole in the bread that goes halfway through.

Scoop out the "hole" and eat as a snack while continuing. This should leave a nice "cup" in which to nestle the egg yolk.

Toast the bread boxes directly on the oven rack for 5 minutes, or until nicely toasted.

While bread is toasting, prepare mushrooms.

Place the butter in a skillet and melt; add mushrooms and sauté over medium heat until done- about 6 minutes.

Add marjoram, season with salt and pepper, and sauté 1 minute longer to meld the flavors.

When bread boxes are toasted (note: the sugar content of pain de mie is high and it browns quickly), remove them and allow to cool a minute or so.

Carefully place an egg yolk in each cup, trying your best not to break it.

Cover each bread box and egg yolk with two slices of cheese, overlapping slightly.

Place on a a greased cookie sheet and bake 5 minutes, or until cheese is beginning to brown an dis bubbly.

To serve, place one box on each of two plates and top with mushrooms. Drizzle with truffle oil and serve immediately.