Make the brownies: Preheat the oven to 350˚. Line a 9-by-13-inch baking pan with foil, leaving a 2-inch overhang on the two short sides; coat with cooking spray. Combine the chocolate and coconut oil in a medium saucepan over medium heat. Cook, stirring, until melted, 3 minutes. Remove from the heat and stir in the granulated sugar, eggs and coconut extract until well combined. Stir in the flour and salt until smooth. Stir in the coconut and nuts. Spread the batter evenly in the prepared pan.
Bake until the brownies are dry on top and a toothpick inserted into the center comes out clean, 30 to 35 minutes. Transfer to a rack and let cool completely in the pan.
Meanwhile, make the icing: Combine the meringue powder, confectioners' sugar, coconut extract and 3 tablespoons water in a large bowl. Beat with a mixer on medium-high speed until stiff and glossy but still spreadable, 2 to 3 minutes. Thin with up to 2 more tablespoons water, if needed.
Lift the brownies out of the pan using the foil overhang. Scrape the icing on top and spread to the edges. Sprinkle with the coconut and white nonpareils; gently press to adhere. Let the icing harden, 1 to 2 hours. Cut into pieces.