Bring the heavy cream, butter, brown sugar, corn syrup and salt to a simmer in a small saucepan over medium heat, whisking occasionally, until the butter melts and the sugar dissolves, about 3 minutes. Whisk in the chocolate and cocoa powder until smooth. Remove from the heat and whisk in the liqueur and vanilla.
Transfer the sauce to a small bowl. Serve warm or at room temperature with cookies for dipping.