In a medium saucepan over low heat, combine the chopped bittersweet chocolate and butter. Stir gently until melted and smooth, being careful not to let it scorch.
Once the chocolate and butter are melted, whisk in the unsweetened cocoa powder and granulated sugar. Continue to stir until fully combined and the mixture is smooth.
Gradually add the heavy whipping cream to the chocolate mixture, stirring constantly. This will create a creamy, luscious sauce. Keep stirring until the sauce is well blended and heated through.
Remove the saucepan from heat and stir in the vanilla extract. This step adds an aromatic depth to your sauce.
Let the sauce cool slightly before transferring it to a storage container. You can use it warm or let it cool completely and store it in the refrigerator for later use.
