Melt butter over medium heat in a small saucepan. Transfer to a large bowl, stir in tahini and salt. Let cool for about 5 minutes, then stir in egg whites.
Meanwhile, beat the 6 egg yolks in an electric mixer for 3 minutes. Add brown sugar and beat for an additional 8 minutes, until the mixture is thick and glossy.
Fold yolk mixture into tahini. Gently stir in flour and cocoa powder. Cover and refrigerate for 1 hour or up to overnight.
Preheat oven to 375F. Generously butter 6 compartments in a 12 cup muffin tin on all sides (I tend to use the 6 in the center to help with even baking). Divide batter evenly among the 6 compartments. Bake for 12-14 minutes, rotating the pan half way through. You'll know they're done when they puff and slightly brown around the edges. Remove from oven and invert immediately onto serving platter or cutting board. Transfer with spatula to individual plates. Sprinkle with pistachios and confectioners sugar to finish.
Recipe by Things I Made Today at https://www.thingsimadetoday.com/2018/06/03/chocolate-tahini-molten-cakes/