Combine 1½ teaspoons water and ¼ teaspoon baking soda in small bowl. In large bowl, toss beef with baking soda mixture and ¼ teaspoon salt until thoroughly combined. Set aside.
Heat oil in 10-inch nonstick skillet over medium heat until shimmering. Add onion and remaining ⅛ teaspoon baking soda and stir to coat. Cook, stirring occasionally, until onion is soft, 3 to 4 minutes. Add garlic and cook, stirring constantly, until fragrant, about 30 seconds. Stir in sugar, paprika, pepper flakes, and remaining ⅛ teaspoon salt and cook, stirring constantly, until paprika is fragrant, about 1 minute. Add tomato paste and cook, stirring constantly, until paste is rust-colored, 3 to 4 minutes.
Add beef and cook, breaking up meat with wooden spoon, until beef is no longer pink, about 5 minutes. Mash beef with potato masher until fine-textured, about 1 minute. Add ketchup, vinegar, and Worcestershire and stir to combine, scraping up any browned bits.
Combine cornstarch and remaining 1½ teaspoons water in small bowl, then pour cornstarch mixture over beef and stir to incorporate. Cook, stirring constantly, until sauce thickens and coats beef, about 1 minute. Season with salt, extra sugar, and extra vinegar to taste. Spoon beef mixture onto buns and serve.