Preheat the oven to 350 degrees F. Spray a 15-in. x 10-in. x 1-in. baking pan with nonstick spray.
PREPARE THE 1ST LAYER:
In a large mixing bowl, use an electric mixer to combine the sugar and butter. Add the eggs and vanilla, and continue to mix until light and fluffy.
In a medium bowl, whisk together the flour, cocoa, baking powder, and salt, and add to the butter mixture. Spread the batter evenly into the prepared pan, and bake 15 to 18 minutes.
PREPARE THE 2ND LAYER:
Sprinkle marshmallows evenly over the first layer and return to the oven for 2 to 3 minutes. Using a knife dipped in water, spread the melted marshmallows evenly over the first layer. Let cool.
PREPARE THE 3RD LAYER:
For the topping, combine the chips, butter and peanut butter in a medium saucepan. Cook over low heat, stirring occasionally, until melted and well-blended. Remove from heat and stir in the Rice Krispies. Spread evenly over bars.