1. In a heatproof bowl, soak the morels in the boiling water until softened, about 20 minutes. Rub them to remove any grit, then lift them out and finely chop.
2. In a small skillet, heat the olive oil. Add the shallot and cook over low heat until softened but not browned, about 4 minutes. Let cool to room temperature.
3. In a medium bowl, using a handheld mixer at medium speed, beat the butter with the morels, shallot, prunes and Madeira until well blended, light and fluffy. Season with salt and pepper. Use at once or scrape the butter into a log shape on plastic wrap and roll it up in the plastic. Refrigerate until ready to use, or freeze for up to 1 month.