Butter six (6-ounce) ramekins and dust with cocoa powder. Place the chocolate and the butter in a large glass bowl and set over a pot of simmering water. Melt and whisk until smooth. Remove the chocolate mixture and let it cool for 5 minutes. Grind the coconut sugar in a coffee grinder or spice mill until it becomes a powder. Add the powdered coconut sugar, arrowroot, eggs, egg yolks and coconut flour to the chocolate mixture and whisk until smooth. Spoon batter evenly into ramekins. Freeze for at least 5 hours or overnight.
Preheat the oven to 450 degrees F and adjust the rack to the middle position. Place the frozen cakes on a baking sheet and bake for 18 minutes. Remove cakes from the oven and let them cool for exactly 2 minutes (while they cool, get out your serving plates and a dinner knife). Run a knife around the edge of the cake and then invert each cake onto a serving dish. Sprinkle with cocoa powder if desired and serve immediately.