This recipe, from the Hershey's web site, makes 15 2-inch cupcakes (without nuts).
1 cup (2 sticks) butter
1/2 cup unsweetened cocoa powder
1 cup packed light brown sugar
1/2 cup granulated sugar
3 large eggs
1 tsp pure vanilla extract
1 cup all-purpose unbleached flour
1-1/3 cups chopped walnuts (optional)
Preheat oven to 350°F. Line muffin pans with 15 paper baking cups (use two muffin pans, and divide the number of muffins equally between them), and set aside.
Place the butter in a large microwave-safe bowl (I use an 8-cup glass measuring cup). Microwave on HIGH for 1-1/2 minutes, or until the butter is melted.
With a wooden spoon or rubber spatula, stir in the cocoa powder until the mixture is smooth. Then, add both sugars, and stir until well blended. Beat in the eggs and vanilla extract. Add the flour and, if you're using them, 1 cup of the walnuts, and stir until the batter is well combined.
Use an ice cream scoop to fill the muffin cups 2/3 full. Sprinkle remaining chopped nuts (optional) over the top of each.
Bake 22-25 minutes, until the tops are cracked and feel firm (the inside will still be moist).
Remove from the oven and let the brownies sit in the muffin pan for 10 minutes. Then, remove to a wire rack, and cool completely.
Can be stored overnight in an airtight container at room temperature.