For the flour, sift more than you need onto a square of waxed paper and spoon lightly into dry measuring cup. Two cups should weigh 7 ounces.
Put the flour into a medium bowl, add the baking soda and salt, and whisk together thoroughly, about 30 seconds, to combine well and to aerate.
Insert metal blade into the work bowl and add the broken up chocolate.
Pulse 3 times for about 1 second each, then process continuously until chocolate is finely chopped, about 1 minute.
With the machine running, pour boiling water through the feed tube and process 30 seconds. Scrape work bowl and process a few seconds more. Mixture should be smooth and chocolate completely dissolved. If necessary, process a few seconds longer.
With the machine on, add the ice water through the feed tube and process 5 seconds.
Add the buttermilk and process 5 seconds.
Transfer liquid to a 2-cup measure with a pouring spout (scrape it thoroughly out of the work bowl) and let cool completely.
Do not wash the work bowl. Re-assemble bowl and metal blade. Put the sugar, eggs, and vanilla into the work bowl.
Process for 1 full minute. Scrape the bowl and add the butter pieces.
Process until smooth, another 1 minute.
Add the chocolate liquid and process for 10 seconds. The batter will be very thin.
Scrape the work bowl well and add dry ingredients on top.
Now here’s the important part. Pulse 4 times in all, very rapidly, a fraction of a second each, only as long as it takes to say on/off, to incorporate the flour. There will probably be a few specks of flour sitting on top of the batter and on the sides of the work bowl. The point is not to over-activate the gluten or the cake will be tough. So go easy.If all the flour has not been incorporated, use the plastic scraper to stir it into the batter.
Scrape the batter into the prepared pan. It will level itself.
Let stand on your counter top 5 to 10 minutes to give the flour particles time to absorb the liquid.
Bake 30 to 35 minutes, until the top of the cake springs back when lightly pressed and the edges of the cake pull away slightly from the sides of the pan. Do not overbake
Cool the cake in its pan on a rack for 10 minutes.
Run a small sharp knife around the sides of the cake to release, and cover with a wire cooling rack. Invert the two and lift off the cake pan. Cover the cake with another wire rack and invert again to cool right side up.