Line a 9x5 inch loaf pan with parchment or waxed paper.
In a heatproof bowl set over a pan of barely simmering water, melt the coconut oil and almond butter together, whisking until smooth. Whisk in the vanilla extract.
Remove from heat and whisk in the powdered sweetener, cocoa powder, collagen, and salt. The mixture will thicken up quite a bit. Whisk in the water until combined.
Using a rubber spatula, work in the coconut flour a tablespoon or two at a time, until the mixture resembles thick cookie dough. (It may be very greasy feeling... don't worry, this will reabsorb as it chills).
Press into the prepared baking pan as evenly as possible and chill until firm, about one hour.
In another heatproof bowl set over a pan of barely simmering water, melt the chopped chocolate until smooth. Use an offset spatula to spread over the chilled bars.
Return to the refrigerator until the chocolate is set, about 20 minutes, before cutting into bars.
Notes
If you don't have collagen or don't want to use it, you can add more coconut flour at the end of the recipe. Start with the 3 tbsp, and keep working it in until you have a thick cookie dough.