Line a heavy baking sheet, not a cookie sheet it should have sides, with a heat proof silicone mat or grease generously with butter. Lay the sheet on a heatproof surface.
In a medium bowl, whisk together the cocoa powder and water until the mixture thickens, then pour through a mesh strainer into a heavy 6 quart non stick pot. Add the sugar, corn syrup, butter, vinegar, baking soda, and salt and whisk to combine. The mixture should not fill more than half the pot as it will boil and rise during cooking. Boil over moderate heat, stirring occasionally during the first 5 to 10 minutes (when mixture is foamy), then do not stir (mixture will bubble vigorously, but settle in the pot). Continue cooking until an instant read thermometer registers 250 degrees (about 15 to 20 minutes), stir in 2 tablespoons of butter until combined, and carefully pour into prepared pan without scraping the pot.
Using a lightly buttered metal pastry scraper or wooden spoon, begin to fold the cooler edges of taffy onto the center (The taffy will become firmer on the edges after a few minutes, but the heat will be concentrated in the center). Taffy will be very soft initially and seem like a thick liquid. Avoid scraping the bottom of the pan.
When taffy is still hot but cool enough to handle, butter your gloved palms generously and gather taffy mass in your hands (you may need to use the scraper to help you). Begin to "pull" taffy in an even thickness by stretching the mass about 1 foot apart in both hands. (taffy may seem very soft and sticky). Bring both ends of taffy back to each other to form a loop and pull, stretching your arms slightly. Twist the end if you like and bring it back to the top. Taffy may feel very soft at first, but it will begin to get harder to pull. Continue pulling until the taffy is lighter in color and has a satin-matte sheen (it will look somewhat like rope) but remains soft and pliable, about 10 minutes (temperatures and humidity may vary and take longer).
When it cools to room temperature, transfer to a clean cutting board and divide into quarters using a heavy knife. Dust lightly with confectioners sugar, roll or stretch the taffy into a log about 3/4 inch wide, and cut into 3/4 inch long pieces. If knife becomes sticky grease knife with butter. Wrap each piece of taffy in waxed paper and store in air tight container. Taffy will stay fresh for 1 week in a container.