In an electric mixer fitted with a paddle, mix the butter until creamy. Add the cooled barley, the breadcrumbs, mustard seeds, mustard powder, Dijon mustard, and grainy mustard and season with salt and pepper; mix just until combined. Scrape the butter onto a sheet of parchment paper, set another paper on top, and roll into a ⅛-inch-thick sheet. Refrigerate until firm, or for up to 3 days.