Set sous vide machine to 53.5C/128F.
In a bowl, mix together 1 tablespoon melted butter, lemon juice, honey, sriracha and garlic.
Salt and pepper the salmon fillet to taste. Sprinkle 3 dashes of cayenne pepper and red chili flakes on top of the salmon. Put the salmon in a freezer bag with garlic lemon butter mixture and rosemary, and remove the air through a vacuum sealer or the displacement method. Drop salmon in the bath for 45 minutes.
Remove bag from bath. Gently take salmon out of the bag, pat dry and lightly re-season with salt and pepper to taste. Discard the rosemary from bag, and set the garlic lemon butter sauce aside.
Heat medium size pan on high and add 1 tablespoon of butter. Sear for 30 to 60 seconds on each side. Remove from pan and place on plate. Lower the heat to medium, add sauce from bag and simmer for 1 minute. Pour the sauce on top of the salmon, and enjoy!
The salmon will be extremely delicate once it finishes cooking in the sous vide. Make sure to handle it gently when removing from bag and searing on the pan.