3 ounces semisweet baking chocolate
1 tablespoon unsalted butter or margarine
1/4 cup peanut butter
2 tablespoons vegetable oil
1 cup firmly packed light brown sugar
1 teaspoon vanilla extract
3/4 cup all-purpose flour
1/4 teaspoon salt
3/4 cup coarsely chopped unsalted peanuts
Preheat oven to 325 degrees. Grease the bottom and sides of a 9-inch square baking pan. In a small saucepan, melt the chocolate with the butter over low heat; stir to blend. Set mixture aside to cool.
In a medium bowl, beat the peanut butter and oil together with an electric mixer until combined. Add the sugar and continue beating until mixture is light and fluffy. Mix in he eggs, one at a time, beating well after each addition. Beat in the vanilla. Beat in the flour and salt until just combined.
In a small bowl, stir together 1/2 cup of the peanut butter batter and the cooled chocolate mixture. Stir the peanuts into the remaining peanut butter batter.
Scrape the peanut butter batter into the prepared baking pan and spread evenly with a spatula. Spoon the chocolate mixture over the batter. With a table knife or metal spatula, make a zigzag pattern through the layers to create a marbled effect. Bake for 35 to 40 minutes or until a cake tester inserted 2 inches from the center comes out almost clean. Cool the brownies in the pan, set on a wire rack.