How to Make It
Toast coriander, fennel, caraway, and black peppercorns in a small skillet over medium, shaking pan, until fragrant, 2 to 3 minutes. Let cool, then grind spices. Mash softened butter with 2 teaspoons kosher salt and toasted spices. Roll butter in parchment; freeze until firm.
Using a sharp knife, score duck skin crosswise at 1/4 -inch intervals. Heat a large skillet over high. Add duck breasts, skin side down, and season with salt and ground black pepper. Reduce heat to low and cook, spooning fat from skillet as it accumulates, until skin is deeply golden and crisp, about 15 minutes. (Reserve fat for another use.) Flip duck breasts and cook to desired degree of doneness, about 7 minutes for medium. Transfer duck breasts to a carving board, skin side up, and let rest 10 minutes.
Slice duck against the grain. Using a box or hand grater, grate some compound butter over duck, allowing the butter to melt into the meat. Divide among 4 plates, and top with additional butter before serving.