Preheat oven to 325°. Line a round 6 inch pan with parchment paper, grease pan and dust with unsweetened cocoa powder. Place in a 13x9 pan and set aside. Bring a kettle of water to a boil.
Using a hand mixer on high speed, beat eggs, coconut sugar, vanilla, and salt until foamy, increased in volume, and lightened in color, about 5 minutes.
Meanwhile, in a heatproof bowl set over a saucepan of simmering water, melt unsweetened chocolate, butter flavored coconut oil, and espresso (if using) until smooth and very warm, stirring once or twice.
Using a large rubber spatula, fold ⅓ of egg mixture into melted chocolate mixture until only a few streaks of egg are visible. Fold in ½ of remaining foam, then last of remaining foam until mixture is completely mixed with no streaks of egg foam remaining.
Scrape batter into prepared pan and smooth top with rubber spatula.
Set 13 x 9 pan on oven rack and pour enough boiling water into pan to come about ½ way up side of pan. Bake until cake has risen just slightly, edges are just beginning to set, and instant read thermometer inserted in center of cake register 140°, about 18-20 minutes.
Remove cake from water and set on wire rack to cool to room temperature. (Run thin knife between edge of cake and cake pan to loosen.)
Remove from pan and place on desired serving tray. Glaze and garnish as desired.