1Preheat oven to 350°F (180°C). Line a baking sheet with parchment paper.
2Whisk together flour, baking powder, baking soda, and cornstarch in a medium bowl. Set aside.
3Cream together brown sugar, granulated sugar, and butter in a large bowl using an electric mixer until light and fluffy.
4Beat in eggs one at a time, followed by vanilla extract and salt. Beat for 3-4 minutes until pale and sugar is well incorporated.
5Gradually mix in the dry ingredients until just combined; do not overmix.
6Stir in chocolate chips and walnuts.
7Scoop 2 tablespoon-sized balls of dough onto the prepared baking sheet, leaving about 4 inches between each cookie. Top with more chocolate chips and walnuts.
8Bake for 13-16 minutes, or until edges are lightly golden brown.
9Remove from oven and let cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.