Preheat oven to 350°F. Grease and flour a 9x13 inch baking pan.
In a medium bowl, combine the flour, baking powder, and salt. Set aside.
In a large bowl, mix together the melted butter and sugar until well blended. Add the eggs one at a time, beating well after each addition. Add the vanilla and mix until combined.
Gradually add the flour mixture to the butter mixture, stirring until just combined. Do not over mix.
Fold in the chocolate chips and walnuts (if desired). Pour into the prepared 9x13 inch baking pan.
Bake in preheated oven for 35-45 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely before cutting into bars.
Interesting Facts
Brownies are believed to have been invented in the United States in the late 1800s.
The first published brownie recipe appeared in the 1896 edition of the Boston Cooking-School Cookbook.
Brownies are a popular dessert in the United States, and can be served plain or with a variety of toppings.