For the horseradish creme fraiche: Whisk together the creme fraiche, horseradish and mustard in a small bowl. Season with salt, and pepper, cover and refrigerate for at least 1 hour to allow flavors to meld.
For the Johnny cakes: Preheat the oven to 300 degrees F. Whisk together the flour, cornmeal, sugar, salt, and baking powder in a medium bowl.
Whisk the egg yolks and milk together in a medium bowl until combined. Whisk the egg whites in a medium bowl until stiff peaks form.
Add the egg-milk mixture to the dry ingredients and mix until just combined. Gently fold in the egg whites, then 2 tablespoons of the melted butter and mix until the batter is just smooth. Let rest for 10 minutes.
Heat a large nonstick saute pan or cast iron pan or griddle over medium-high heat. Brush with some of the remaining melted butter. Spoon 2 heaping tablespoons of the batter into the pan for each johnnycake and using a small offset spatula, spread the batter slightly to even out the top. Cook until lightly golden brown on each side, about 1 1/2 minutes per side. Remove to a baking sheet and keep warm in the oven until ready to serve. Top each cake with a small dollop of the creme fraiche and some of the trout. Garnish with parsley leaves and a sprinkling of chopped chives.