Preheat oven to 350°F. Line an 8x8* square pan with foil or parchment paper, leaving a couple inches of overhang on two sides (this will allow you to lift the whole tray of brownies out of the pan for easy removal). Grease the foil/parchment.
Combine granulated sugar and hot butter in a large bowl. Stir it briefly and then allow it to sit undisturbed for 5 minutes. The heat from the butter will begin to dissolve the sugar (this helps make a shiny/crackly crust). The sugar will not fully dissolve. After 5 minutes, whisk the butter and sugar vigorously for 30 seconds. Add eggs, water, and vanilla extract, whisking again for about 30 seconds. Tap all the excess batter off the whisk and remove the whisk from the bowl.
Sprinkle cocoa powder, powdered sugar, flour, salt, and chocolate chips over the wet ingredients. Use a rubber/silicone spatula or wooden spoon to stir the mixture until it is glossy and well combined. Gently stir in the M&M's.
Pour the batter into the prepared pan and smooth the top. If desired, press a few additional M&M's and chocolate chips onto the top of the batter. Bake for 28-33 minutes (begin checking at 28 minutes). The brownies are done when a toothpick inserted into the center comes out without raw batter on it; moist fudgy crumbs are OK. Be careful not to over bake. Remove from the oven and place on a cooling rack to cool completely.
*The 8x8 square baking dish should have straight sides. An 8x8 baking dish with angled sides will take longer to bake.