In a 1-quart stainless steel saucepan, combine cream, water, corn syrup, brown sugar, cocoa powder and salt over medium-low heat. Bring it to a simmer gently while whisking frequently until sugar is dissolved.
Remove from the heat and add the chopped chocolate. Let the chocolate melt gently for a minute, and then return to medium-low heat.
Once it simmers, increase heat to medium and let boil gently for 3-5 minutes while whisking constantly. Do not rush this stage or the sauce will have a more grainy texture rather than smooth. Let it bubble at a low boil, but do not boil vigorously or the fat can separate.
For a thinner sauce boil for 3 minutes, for a thicker fudgier sauce cook for 5 minutes. It should be smooth and glossy at this point but sure not to taste it as it is PIPING hot. Make sure to stir constantly and get into the bottom edges of the pan as the chocolate can burn easily. The mixture will be thick and luscious if cooked properly. You’ll notice that the bitter, roasted notes will become more pronounced after the sauce has cooked, making it taste even more chocolaty.
Remove pan from heat and stir in vanilla extract. Let cool about 15 minutes and serve warm or pour it immediately into a sterilized heatproof mason jar, seal tightly and let it cool completely at room temperature. Store it in the refrigerator for up to 3 weeks. Reheat very, very gently before serving over your favorite ice cream.