Begin by poaching the salmon. Fill a small saucepan with 2 inches of water and add a generous pinch of salt. Bring to a simmer over high heat. Add the salmon fillet and turn the heat down to keep it at a simmer. Cook for 5 to 8 minutes or just until the salmon is cooked through. When fully cooked, remove the salmon with a slotted spoon and set aside to cool completely.
While the salmon is cooling, prep the other ingredients. Finely chop the shallot and chives. Juice the lemon and set aside.
In a mixing bowl, flake the salmon with a fork. Add the softened butter and mayonnaise and work into the salmon with a spatula. Add the shallots, chives, lemon juice, and a generous pinch of pepper. Mix together well with a spatula and then taste for seasoning. Add kosher salt if needed.
Transfer the rillettes to a serving bowl or jar. Serve with sliced baguette and enjoy!