In a bowl, soak the livers in the milk for 2 hours. Drain well.
In a large saute pan or skillet, melt 4 tablespoons of the butter over medium-high heat. Add the onions and cook, stirring, until soft, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds. Add the chicken livers, 1 tablespoon of the peppercorns, the bay leaves, thyme, salt, and pepper and cook, stirring, until the livers are browned on the outside and still slightly pink on the inside, about 5 minutes. Add the Cognac and cook until most of the liquid is evaporated and the livers are cooked through but still tender.
Remove from the heat and let cool slightly. Discard the bay leaves.
In a food processor, puree the liver mixture. Add the remaining butter in pieces and pulse to blend. Fold in the remaining 1 tablespoon peppercorns and adjust the seasoning, to taste.
Pack the pate into 6 individual ramekins or small molds, about 4 ounces each. Cover with plastic and refrigerate until firm, at least 6 hours.
To serve, place the ramekins on individual plates. Garnish the tops with parsley and surround with croutons. Serve with cornichons on the side.