Place butter pieces and sugar in a bowl and set it over a bowl of hot water to melt.
Remove from heat and add chopped chocolate, melting it with the residual heat.
Add room temperature eggs and vanilla in two additions, mixing with a whisk each time.
In a separate bowl, mix coconut flour, baking powder, cocoa powder, and tapioca starch well.
Sift the dry ingredients into the butter-chocolate mixture in two portions, whisking gently each time.
Heat cacao nibs in a pan for 30 seconds, then mix into the batter.
Bake in a preheated oven at 170°C for 8~9 minutes (preheat to 180°C beforehand).
Let cool in the pan, then freeze for 3 hours before cutting and serving from frozen.