Preheat the oven to 375 degrees.
Line a large rimmed cookie sheet with foil. If your fish is very large you may need two sheets as you will be folding it up and pinching the foil closed around the salmon.
In a small bowl, stir together the mustard, chili powder, smoked paprika, brown sugar, and salt. It should form a very thick paste.
Put the salmon on the foil and using the back of a spoon, spread the entire fish with the mustard paste.
Pull the foil up and over the salmon, being careful to not lay the foil on the salmon as the paste will stick to it, and pinch the foil closed like a packet.
Bake for 14-15 minutes.
Remove from the oven, avert your face and carefully open the foil being sure to not shoot steam in your face.
Turn the oven to broil.
Drizzle the maple syrup all over the salmon.
Place the salmon back in the oven under the broiler for 2-3 minutes.
Remove from the oven and serve immediately.