Stir the flour, baking powder, baking soda and salt together in a medium bowl; set aside.
Cream the butter and both sugars in a large mixing bowl using high speed of an electric mixer; Scrape sides of bowl and beat in the vanilla. Add the egg and egg yolk and beat just until eggs are incorporated. Add the flour mixture and stir until absorbed. Add oats and stir just until blended. Stir in the nuts and chocolate chips.
Shape the dough into about 18 large balls (they should weigh a little less than 3 oz each). Press the balls down slightly so you have disks. Lay them on dinner plates or a small cutting board, cover with plastic wrap and chill for an hour or until ready to bake.
Chill the dough for one hour or more.
Preheat oven to 325 degrees F. Have ready two or three ungreased cookie sheets.
Arrange the dough disks on a large baking sheet spacing 3 inches apart. Bake at 325 for 15-17 minutes or until cookies are brown around edges but still soft in the middle. Allow them to sit on the cookie sheet for 3 minutes, then transfer to a rack to cool.
Makes 18 large cookies.