Line a 9x13-inch pan with parchment, leaving overhang for removal.
Microwave peanut butter and butter in a large microwave-safe bowl on high 2 minutes; stir until completely smooth and combined.
Stir in vanilla and salt.
Mix in powdered sugar one cup at a time, stirring well after each addition until smooth and creamy.
Spread fudge evenly in pan with a spatula; smooth top.
Refrigerate at least 2 hours until firm; lift out by parchment and cut into 1-inch squares.
Store airtight in refrigerator up to 2 weeks or freeze up to 3 months.