Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking pan.
In a large saucepan, melt 1 cup butter. Remove from heat, and stir in 2 cups sugar, 4 eggs, and 1 teaspoon vanilla. Beat in 1 cup flour, 1/2 cup cocoa, and 1/2 teaspoon salt. Spread mixture into the prepared pan.
In a small saucepan, melt 1 cup peanut butter. Drizzle the peanut butter over the brownie layer. Using a knife, swirl the peanut butter into the brownie layer.
Bake for 25 to 30 minutes in the preheated oven. Cool before cutting into bars.
Interesting Facts
Peanut butter brownies have been popular in the United States since the 1920s.
Peanut butter brownies are an American classic, but they are also enjoyed in many other countries around the world.
The combination of chocolate and peanut butter is one of the most popular flavor combinations in the world.