Prepare the Brownies: Preheat oven to 350°F. Lightly coat a 13- x 9-inch baking pan with baking spray. Line with parchment paper, leaving a 2-inch overhang on 2 sides.

Microwave butter and 1 ½ cups of the chocolate chips in a large microwavable bowl on HIGH until melted and smooth, about 90 seconds, stopping to stir every 30 seconds. Stir in granulated sugar and vanilla until smooth. Stir in eggs until smooth. Whisk together flour, cocoa, baking powder, and salt in a small bowl, and stir into chocolate mixture until just combined. Stir in remaining ½ cup chocolate chips. Spread mixture evenly in prepared baking pan. Bake in preheated oven until just set, 26 to 28 minutes. Transfer to a wire rack; cool completely, about 1 hour.

Meanwhile, prepare the Marshmallow Topping: Place ⅓ cup of the cold water in bowl of a stand mixer fitted with a whisk attachment, and sprinkle with gelatin. Stir together granulated sugar, corn syrup, salt, and remaining ⅓ cup cold water in a small saucepan. Cook over medium-high, stirring often, until a candy thermometer registers 240°F, 9 to 12 minutes (reduce heat if needed to prevent boiling over).

Gradually add hot sugar mixture to gelatin mixture in stand mixer bowl. Beat on medium-low speed, gradually increasing mixer speed to high, until bottom of bowl is slightly warm and mixture is very thick, 6 to 7 minutes. Stir in vanilla.

Working quickly, spread Marshmallow Topping over cooled Brownies; smooth with an offset spatula lightly coated with cooking spray. Stir together powdered sugar and cocoa in a small bowl. Using a fine mesh strainer, lightly dust Marshmallow Topping with powdered sugar mixture.

Let stand, uncovered, in a cool, dry place until set and dried, at least 8 hours or up to 24 hours. Slice using a knife dusted with powdered sugar after each cut.