Preheat oven to 350 degrees.
Butter a 9″x9″ square baking pan.
In a double boiler over low heat, melt butter and chocolate chips together and remove from heat.
In a separate bowl, combine dry ingredients.
When the chocolate/butter mixture has cooled a little whisk in the eggs and vanilla.
Note: it is important to cool the chocolate/butter mixture or the eggs will cook.
Add the dry ingredients and whisk to combine.
Stir in the chopped walnuts.
Pour batter into prepared baking pan.
Bake for 40-45 minutes or until a knife inserted into center comes out clean.