Preheat oven to 350°. Line a 13x9-in. baking pan with foil, letting ends extend up sides; coat foil with cooking spray.
Microwave butter, sugar and water on high just until mixture comes to a boil, 3-4 minutes; stir until blended. Stir in chocolate chips until melted. Whisk in eggs, 1 at a time, stirring well after each addition. Whisk in vanilla. Stir in flour and baking powder.
Spread into prepared pan. Bake until a toothpick inserted in center comes out clean, 18-20 minutes. Cool 30 minutes.
For topping, combine sugar, butter and milk in a large saucepan; bring to a boil, stirring constantly, over medium heat. Boil 5 minutes, stirring frequently. Stir in marshmallow creme, 1/4 cup peanut butter and vanilla; pour over brownies. Sprinkle with 2 cups peanuts.
In a small saucepan, combine caramels and water; cook, stirring, over medium-high heat until blended. Pour over peanuts.
Microwave chocolate chips on high until softened, about 1 minute. Stir in remaining peanut butter until smooth; pour over caramel layer. Chop remaining peanuts; sprinkle on top. Refrigerate at least 1 hour.
Lifting with foil, remove brownies from pan. Cut into bars. Store in an airtight container in the refrigerator.
Test Kitchen tips
Get ready for a delicious mouthful. These bars are show-stoppingly rich, so cut them small.
Chilling the brownies in the pan makes them easier to lift out and cut, so don't skip that step.
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