To make the blue cheese dressing: In a bowl, break up blue cheese and whisk in buttermilk. Add the remainder of the ingredients and mix to a chunky consistency. Add salt and pepper to taste.
For the sammies: Spread blue cheese dressing evenly on each slice of bread.
Layer chicken, watercress, celery and walnuts and close the sandwiches up.
Heat your Panini maker or sandwich press and butter each side of the press (if you don't have a sandwich press, butter the top of the bread and weight down the sammies with a cast iron skillet or a garden brick wrapped in foil.
Place the sandwiches inside, press down and grill until the bread is flat and browned, approximately 10 minutes. Serve warm.