How to Make It
In a large skillet, cook the mustard seeds over moderately high heat until toasted, about 1 minute. Transfer to a plate. Heat 4 tablespoons of the butter in the skillet. Add the shallots and cook over moderate heat until softened. Add the livers, season with salt and pepper and cook until barely pink inside, about 8 minutes. Add the cognac and simmer for 1 minute. Scrape into a food processor; let cool.
Add 1 1/2 sticks of the butter to the livers and puree until smooth. Transfer to a bowl. Fold in 3 teaspoons of the mustard seeds, 3 tablespoons of the parsley and 8 of the sliced caperberries and season with 1/2 teaspoon each of salt and pepper. Spread the mousse in a 6-cup soufflé dish and top with the remaining parsley, mustard seeds and caper berries.
In a small bowl, gently melt the remaining 4 tablespoons of butter in the microwave. Skim the foam from the surface. Spoon the clear butter evenly over the mousse, leaving behind the milk solids; tilt to distribute the butter. Cover and refrigerate overnight.