Preheat oven to 350°F.
In a large, deep saucepan, melt butter on the stove. Remove from heat and use a wooden spoon to stir in sugar and vanilla until smooth. Add eggs, one at a time, and beat in using spoon. Add flour, baking powder and salt, and mix well to form smooth batter. Pour batter into an ungreased non-stick
9" x 13" pan (you could line with parchment paper), or a greased and papered regular baking pan. (The batter will stick to the paper, so be careful spreading it around.) Toss together the chocolate chips and walnuts, then spread them evenly over the batter. Very lightly press them into the top of the batter. Bake for 25 minutes, until the batter is cooked and the top is slightly browned (the edges will brown first).
Let cool completely before cutting into squares. Makes 25-30 squares.