Set the Sous Vide Cooker to 185ºF (85ºC)
Place asparagus in a single layer in a large zipper lock or vacuum seal bag. Add the butter and a pinch of salt. Seal the bag using the water immersion technique or a vacuum sealer on the dry setting.
Place the bag in the water bath and set the timer for 10 to 12 minutes, depending on the thickness of the asparagus.
When the timer goes off and asparagus are tender, remove the bag from the water bath. Remove the asparagus spears from the bag and arrange on a serving plate. Spoon some of the butter from the bag over the spears. Garnish with sea salt and mint leaves. Serve.