Preheat oven to 350ºF or 325ºF Convection
Line bottom of an 8" x 8" square or 9" x 13" baking pan with parchment paper. In a stand or hand mixer, cream butter and sugar together on medium speed until fluffy, about 3 minutes. Reduce speed to low and add an egg at a time to sugar mixture, incorporating fully with each egg. Add vanilla and mix. Sift dry ingredients into a bowl (flour, salt and baking powder). Add dry ingredients to butter/sugar mixture on low speed, until incorporated and blended. Fold in chocolate pieces. Fill baking pan and spread batter evenly. Bake at 350˚F for 35-40 minutes (or 25-30 minutes at 325ºF if using convection). Remove from oven and cool slightly before cutting into squares.