Fill a bowl large enough to fit the pan you're cooking the sugar in with ice and water. Set 2-3 ice cubes aside.
Put the 9 ounces of sugar in a heavy bottomed saucepan. Add enough water just to get everything wet and stir well.
Heat over medium high heat until boiling. It's okay to stir at this point. Once the sugar is boiling, put the lid on for 2 minutes to wash down any sugar crystals that might be hanging out on the sides of the pan.
Remove the lid and watch carefully as the sugar cooks. It will turn a pale honey color and then begin to deepen. Cook the sugar until it is a deep reddish brown. It will smoke, and that's okay. Just be ready to act fast.
As soon as the caramel is the color you want, remove it from the heat and set the pan in the ice bath. Carefully toss the reserved ice cubes into the caramel and swirl the pan. All the foolishness is to make sure you arrest the cooking at exactly the point you want.
Check the temperature of the caramel. You want it to be around 140F.
Pour in the buttermilk. The caramel will be hard at this point and you will be sad. It's okay. All will be well. Set the pan over medium-low heat and reheat, stirring, to melt the caramel into the buttermilk. Don't let the mixture boil or the buttermilk will curdle, so just take it slow and use a spatula to scrape up the melting caramel from the bottom of the pan and stir it into the buttermilk. This will take about 10 minutes. Keep the mixture around 165F or so and then cool it to below 140F before proceeding.
Whisk together the eggs, flour, and cocoa powder. Whisk them really well to avoid lumps.
Whisk the egg mixture into the caramel mixture. Whisk in the melted butter, espresso powder, vanilla, and salt.
Taste the filling. Add additional granulated sugar if you would like it a bit sweeter. If you don't cook your caramel as dark as I cooked mine, you might not need extra, but it's up to you. I think I ended up adding in about 5 extra Tablespoons of sugar.
Pour the filling into the blind-baked pie shell. Bake at 400F for 10 minutes. Reduce the heat to 375, loosely tent the pie with foil and bake until the pie wiggles but doesn't slosh in the center, about 30-35 minutes. Check the temperature in the center of the pie. It should be about 165F.
Remove pie from the oven and set on a rack to cool. Serve barely warm. Refrigerate the leftovers. Pie is good cold or gently rewarmed in the microwave on low power for a few seconds.