Position rack in middle of oven and preheat to 325°. Generously butter an 18x13 rimmed baking sheet.
Melt the butter in microwave or on stovetop.
In a medium bowl whisk together sugar, cocoa, vanilla and salt. Pour the melted butter over the mixture and stir until smooth. Add the eggs, one at a time, stirring vigorously until batter is thick, shiny and smooth. I like to use a heavy wooden spoon. Stir in the flour until well blended then beat vigorously, another 30-40 strokes
Spread batter evenly onto baking sheet in thin, even layer. Push it all the way to the corners. It will seem too thin but the batter will rise.Add your toppings, pressing in firmly. I used about 1/2 cup peanut butter and swirled it through the batter. Then pressing lightly with wet hands I also used mini marshmallows and walnuts for a rocky road as well as dried cherries and almonds. You can get as creative as you want. This is a great activity for a kids or teens party.
Bake until firm to the touch and tester comes out clean, 18-20 minutes. Let cool completely then cut into pieces and serve.
I did one batch and divided it into thirds to try different toppings. Have fun!