Heat a large skillet over medium high heat. Add ground beef and start to brown, breaking it up as you stir. Add brown sugar and spice seasoning, stirring to mix in.
When ground beef has browned, add onion, reduce heat to medium and cook onions until translucent, about 5 minutes.
Stir in mustard, vinegar and Worcestershire sauce, tomato sauce and tomato paste, stirring to combine. Reduce heat to simmer and cook 5-8 minutes longer. Sauce should thicken up and meat mixture is tight.
Butter the buns and toast until golden brown under broiler.
Using a ice cream scoop or spoon pile meat onto toasted bun bottoms and cover with bun tops.