Preheat and prep. Get your oven to 350°F and line a baking sheet with parchment paper.
Mix the dry stuff. In a medium bowl, whisk together flour, baking powder, baking soda, and cornstarch. Set this aside and forget about it for now.
Cream like you mean it. In a large bowl, cream together brown sugar, granulated sugar, and room temperature butter until light and fluffy. This should take about 3-4 minutes with an electric mixer.
Add the wet ingredients. Beat in eggs one at a time, then vanilla extract. Keep mixing for 3-4 minutes until the batter turns pale and the sugar is well incorporated. Add the pinch of salt.
Bring it together. Gradually mix in the dry ingredients until just combined. Don't overmix—once you stop seeing dry flour, stop mixing.
Chocolate time. Stir in both types of chocolate. This is where the magic happens—those unstabilized dark chocolate pieces are about to make your cookies legendary.
Scoop and space. Drop 3-4 tablespoon-sized balls of dough onto your prepared baking sheet. Leave them slightly rough and rustic-looking—those peaks and valleys will create amazing texture. Space them 4 inches apart (4 cookies per pan) because these babies spread. Top with a few extra chocolate chips because we're not here to be subtle.
Bake low and slow. Bake for 13-16 minutes on the middle rack, until the edges are lightly golden brown. The centers might look slightly underdone, but they'll finish cooking on the hot pan.
The waiting game. Let them cool on the baking sheet for 5 minutes before transferring to a wire rack. This isn't optional—move them too early and they'll fall apart.