Peanut Butter Mississippi Mud Bars

Makes 12 bars.

8 tablespoons (1 stick) unsalted butter, softened
1 cup firmly packed light brown sugar
1/2 cup creamy peanut butter
1 large egg, at room temperature
1 tablespoon pure vanilla extract
1 cup all-purpose flour
1 teaspoon baking soda
1/4 teaspoon salt
8 ounces white chocolate, coarsely chopped
and divided into two 4-ounce portions
8 ounces semisweet chocolate, coarsely chopped
    and divided into two 4-ounce portions
1 cup coarsely chopped pecans, toasted
    and divided into two 1/2-cup portions

Heat the oven to 325 degrees F. Butter the baking pan, line it with aluminum foil and butter the foil.

Put the butter and sugar in a large bowl and blend until smooth using the electric mixer on medium speed. Beat in the peanut butter until smooth.

Stir the egg and vanilla extract together in a bowl and add to the peanut butter mixture until combined.

In a small bowl, whisk together the flour, baking soda and salt. Add to the butter mixture and combine well.

Stir in 1 portion each of the white and semisweet chocolates and combine. Add 1 portion of the pecans and mix until combined.

Spread the batter evenly in the prepared pan and bake for 30 to 35 minutes, or until the center is soft but no longer runny. Remove the pan from the oven and immediately sprinkle the remaining white and semisweet chocolates over the top. Cover the pan with aluminum foil and set aside to let the chocolates melt.

When melted, swirl the chocolates together with the tip of a sharp knife to make a marbleized effect. Sprinkle the remaining pecans over the top. Let the bars cool completely in the pan on a wire rack.

To serve, cut into 2-inch squares and arrange on a serving plate.