1. Preheat the oven to 175° Celsius. Line a 9-by-13 inch baking pan with aluminium foil, and spray with cooking spray.
2. Stir together the butter, sugar, eggs and vanilla in a medium bowl. Add the flour, hot chocolate mix, salt and baking powder and stir until just combined. Stir in half the chocolate chips. Spread the mixture in the prepared baking pan.
3. Bake until the brownies begin to pull away from the edges of the pan and are set in the center, 30 to 35 minutes. Remove from the oven, immediately sprinkle with the remaining chocolate chips, and let stand until they are very soft, about 5 minutes. Spread the melted chips evenly with a spatula, and let cool for about 10 minutes. Use the foil to lift the brownies out of the pan, and let cool completely on a rack. Cut into 24 squares, and top each with 3 mini marshmallows.
For best results, don't use hot chocolate or cocoa mixes that contain powdered milk or other dairy products, which can change the structure and texture of the brownies.