Melt the butter in a medium frying pan over medium-high heat. When hot, add the sliced brown mushrooms and cook, stirring occasionally, until browned and no liquid remains, 5 minutes or more. Season with 3/4 teaspoon salt and several grinds of pepper. Add 2 tablespoons of the sherry to the pan. Cook until all the liquid has evaporated, about 1 minute. Add the reserved porcini, 1 cup of the reserved soaking liquid, the cream, and the remaining sherry. Cook, stirring, until the sauce thickens very slightly, about 5 minutes.