Make the brownies: Preheat the oven to 350˚. Line a 9-by-13-inch baking pan with foil, leaving a 2-inch overhang on all sides; coat with cooking spray. Stir together the flour, cocoa powder, baking powder and salt in a medium bowl. Put the chocolate in a microwave-safe bowl and microwave in 30-second intervals, stirring, until smooth; let cool slightly.
Combine the butter, granulated sugar, brown sugar and water in a medium saucepan over low heat. Cook, stirring, until the butter is melted. Off the heat, whisk in the melted chocolate, then the eggs, one at a time, then the vanilla. Whisk in the flour mixture until just combined. Spread the batter evenly in the prepared pan.
Bake until a toothpick inserted into the center of the brownies comes out clean, about 25 minutes. Transfer to a rack and let cool 10 minutes in the pan. Carefully remove from the pan using the foil overhang; transfer the brownies to the rack to cool completely.
Meanwhile, make the frosting: Beat the butter in a large bowl with a mixer on high speed until light, about 1 minute. Add the confectioners' sugar, cocoa powder, milk, vanilla and salt. Reduce the speed to low and beat until combined. Increase the speed to high and beat until light and fluffy, about 5 minutes, thinning with 1 to 2 tablespoons more milk if needed. Spread the frosting on the brownies. Cut into pieces, then decorate with candy eyeballs and sprinkles.