Ingredients

• Chocolate batter:
• 3/4 cup (1 1/2 sticks) unsalted butter, cut into bits
• 3 ounces unsweetened chocolate, coarsely chopped
• 1 1/4 cups sugar
• 2 eggs, at room temperature
• 1 1/2 teaspoons vanilla extract
• 1/4 teaspoon salt
• 1 cup all-purpose flour
• Peanut butter batter:
• 3/4 cup creamy peanut butter
• 1/2 cup sugar
• 1 egg
• 3/4 teaspoon vanilla extract
• 1/4 cup milk
• 2 tablespoons all-purpose flour

Instructions

1. Preheat oven to 350 degrees F. Spray a 9-by-13-inch baking pan lightly with nonstick cooking spray, or use a nonstick pan.
2. To make chocolate batter: Place butter and chocolate in a microwave-proof mixing bowl or in the top of a double boiler. Microwave on medium-high (80 percent) or heat over hot water, stirring occasionally, until melted and smooth.
3. Beat in sugar on medium speed until well blended. Beat in eggs, one at a time, then vanilla and salt. Stir in flour. Pour batter into prepared pan.
4. To make the peanut butter batter: In another bowl, beat peanut butter with sugar, egg, vanilla, and milk until smooth.
5. Drop peanut butter batter by large spoonfuls over chocolate batter. With a butter knife, “cut” the peanut butter batter into the chocolate batter, lifting the chocolate batter and swirling to make a marbled pattern.
6. Bake in the center of the oven for 25 to 35 minutes, or until a tester inserted near the center comes out clean. Let cool in the pan, then cut into bars.