Mushroom powder and margarine contain salt. The total amount of salt should not exceed 6 g. Melt the butter or margarine in a water bath. DO NOT heat above 45 degrees, let cool to 35 degrees. Knead the ingredients into a smooth dough in 15 minutes, kneading in the butter or margarine at the end. Let rise in a warm place, covered with a damp cloth, for 8-12 hours. The long fermentation time releases water and the dough becomes supple and softer.
In the meantime, line the baking sheet with parchment paper.
Knead the opened dough for 2 minutes. Shape 4 breads of equal weight and place on the baking sheet. Brush with the olive oil. Cover the raw ciabattino and let rise for another 4 - 6 hours.
Preheat the oven to 220 degrees bottom heat. Half-fill a shallow, heat-resistant bowl with hot water and place on the lowest level. Bake the ciabattino on the middle rack in 30-35 minutes.
Take out of the oven, cool a little and use immediately. Otherwise wrap in plastic wrap and store in the refrigerator. Consume or freeze within 4 days.