Pour sugar in a large saucepan. Place over medium heat, whisking constantly until all the sugar has melted and color is a medium-amber color. Sugar will clump, but don't worry! Those clumps will melt, so just keep stirring.
Remove from heat and whisk in butter and heavy cream. Whisk until cohesive.
Whisk in vanilla, salt, and cinnamon. Let cool slightly to thicken.