Preheat the oven to 175°C. Line a 9-by-13-inch baking tin with aluminum foil and spray with nonstick cooking spray.
Stir together the sugar, butter, vanilla and eggs in a medium bowl. Add the flour, cocoa powder, salt and baking powder all at once and stir until just combined. Stir in half of the chocolate chips and spread the mixture in the prepared tin.
Bake until the brownies begin to pull away from the edge of the pan and set in the center, 30 to 35 minutes. Immediately sprinkle with the remaining semisweet chocolate chips and let stand until the chips are melted, about 5 minutes. Spread the melted chips evenly over the top of the brownies with a spatula. Let the chocolate cool for about 10 minutes and then sprinkle with whatever combination of toppings you choose, pressing the toppings gently into the chocolate to adhere. Lift the brownies out of the pan with the foil and cool completely on a rack. Cut into 16 squares and serve.