Clean the chicken livers by removing any connective tissue, sinew, or green spots. Pat them completely dry with paper towels. Cut into uniform pieces for even cooking.

Finely mince the shallots and garlic. Measure out all ingredients before beginning to cook.

Heat butter in a large skillet over medium-high heat until foaming. Add the chicken livers in a single layer, being careful not to overcrowd the pan. Sauté for 3-4 minutes on the first side until golden brown.

Flip the livers and add the shallots, garlic, and fresh thyme leaves. Continue cooking for another 3-4 minutes until livers are cooked through but still slightly pink in the center. They should reach an internal temperature of 165°F.

Remove from heat and add cognac to the pan, scraping up any browned bits from the bottom. Let the mixture cool for 10 minutes.

Transfer the liver mixture to a food processor or blender. Add the remaining softened butter, heavy cream, salt, black pepper, and a pinch of nutmeg. Process until completely smooth and silky, about 2-3 minutes, scraping down the sides as needed.

Taste and adjust seasoning if necessary. The mixture should be well-seasoned as chilling will mute the flavors slightly.

Transfer the mousse to ramekins or a serving dish. Smooth the top with a spatula. Optional: Pour a thin layer of melted butter over the top to seal and preserve.

Cover with plastic wrap and refrigerate for at least 4 hours or overnight to allow flavors to meld and texture to firm up.

Remove from refrigerator 30 minutes before serving to bring to room temperature. Serve with toasted bread, crackers, or cornichons.