Combine the flour, baking powder and salt in a large bowl.
Combine butter, sugar and brown sugar in a large bowl. Beat with an electric mixer at high speed until light and fluffy.
Beat eggs one at a time. Stir in vanilla extract. Add flour mixture gradually to butter mixture, beating well after each addition.
Fold in the chocolate chips and walnuts. Cover dough and chill in refrigerator for 2 hours or longer.
Preheat oven to 170c. Grease baking sheets.
Drop the dough by tablespoons onto prepared baking sheets. Flatten dough slightly with a fork.
Bake in preheated oven until browned, about 15 minutes. Let cookies cool on the baking sheets for 5 minutes, then remove to wire racks to cool completely.