1In a medium bowl, whisk together flour, baking powder, baking soda, and cornstarch. Set aside.
2In a large bowl, cream together brown sugar, granulated sugar, and butter until light and fluffy.
3Beat in eggs one at a time, followed by vanilla extract. Beat for 3-4 minutes until pale and sugar is well incorporated.
4Gradually mix in the dry ingredients until just combined, being careful not to overmix.
5Stir in chocolate chips and walnuts.
6Scoop 2 tablespoon-sized balls of dough onto the prepared baking sheet, leaving about 4 inches of space between each cookie. Top it with more chocolate chips and walnuts.
7Bake at 350°F (180°C) for 13-16 minutes, or until edges are lightly golden brown.
8Remove from oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.