Combine chicken livers and milk in a bowl. Let soak in the fridge, about 2 hours.
Melt 2 tablespoons butter in a saucepan over medium heat. Add onion; cook and stir until soft, about 5 minutes. Add 1 1/2 teaspoon black peppercorns, garlic, chilli flakes and thyme; cook and stir until fragrant, 1 to 2 minutes.
Remove chicken livers from the milk with a slotted spoon; add to the saucepan with balsamic vinegar and bay leaves. Season with salt and pepper. Cook until chicken livers are tender and no longer pink, 10 to 12 minutes. Remove from heat and cool slightly, 5 to 10 minutes.
Transfer chicken liver mixture to a food processor. Add remaining 1 1/2 teaspoons black peppercorns and whiskey; blend until smooth. Add remaining 6 tablespoons of butter and pulse until fully incorporated into the mixture.
Divide blended mixture evenly among 6 ramekins. Cover with cling film and refrigerate until firm, about 6 hours.
For longer storage, cover pate with a thin layer of melted butter, cover and refrigerate or freeze.