Let the butter melt, add the flour, sweat for a few minutes, gradually deglaze with the milk, simmer until the sauce has thickened, stirring constantly. Let cool a little, mix in the Parmesan.
Roughly dice the salmon fillet, add the egg yolks, salt and a little pepper. Process quickly with the hand blender to a homogeneous mass. The ingredients should be as cold as possible. Mix in the cooled bechamel sauce and the smoked salmon cut into fine strips. Season to taste and refrigerate until further processing.
Beat the egg whites with two pinches each of salt and baking powder until stiff. Stir one third into the salmon mixture, then carefully fold in the remaining egg whites.
Butter a soufflé mold (capacity approx. 1.8 lt.) Or spray it with baking release paste, distribute the mixture in it. At least 2 cm should remain free up to the upper edge. Place the mold on a baking sheet, put a few cups of cold water on the sheet. Bake at 190 ° C (top / bottom heat, no convection!) For 40 - 45 minutes until the soufflé is golden brown. Never open the oven during baking, otherwise the soufflé will collapse. Serve immediately.