Heat a large, heavy bottom skillet over medium high heat and add butter to melt.
Add ground beef, stirring frequently and breaking into chunks as it cooks. Cook for 5 - 8 minutes or until well browned. Remove meat from pan and reserve all of the ground beef drippings (do not discard any of the fat, it is important to use it to sauté the vegetables in the next step. Without it you will end up with dry and crumbly filling).
Add onion and bell pepper to the skillet and cook for 2 - 3 minutes until soft.
Add garlic and cook for 30-seconds or until fragrant.
Add the brown sugar, chili powder, dry mustard, red pepper flakes, black pepper, kosher salt, and stir until completely combined. Continue to cook for an additional minute or two to toast the spices.
Add the tomato paste to the skillet, stirring to combine and sauté for 1 - 2 minutes until it starts to caramelize and stick to the bottom of the pan.
Add the beef back to the skillet and stir.
Add the ketchup, water, Worcestershire sauce, and hot sauce, stirring to incorporate. Bring sauce to a boil, then reduce heat to medium-low and simmer for 10 - 15 minutes until thickened.
Sloppy Joe Assembly
Spread a thin layer of mayonnaise on the bottom of the toasted buns. Optionally top with bread and butter pickle slices, followed by several tablespoons of the sloppy joe filling.