Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius).

For the brownies, place all the ingredients directly inside a nine- by 13-inch (20- by 30-centimeter) baking pan. Stir together with a fork or a whisk until well-combined.

Place pan in oven and bake for 40 minutes or until toothpick inserted comes out clean (it’s fine if there are two to three crumbs attached). Immediately sprinkle with mini marshmallows and return to oven for one to two minutes, or until marshmallows begin to puff. Remove from oven and set aside to cool a bit.

Meanwhile, place the vanilla, corn syrup, peanut butter, sugar, and margarine in a heavy-bottomed saucepan and cook over medium heat, stirring until the mixture melts and begins to simmer. Remove from heat and stir in peanuts and crisp rice; immediately spread over the marshmallows and allow to cool.

Cut into bars with a sharp knife.