Heat the oven to 375°F (190°C). Line a large baking sheet with parchment paper and set aside.

Whisk together the all-purpose flour, baking powder, baking soda, and salt in a medium bowl until evenly combined.

In a large bowl or the bowl of a stand mixer, work the softened butter with the brown and granulated sugars plus the vanilla. Cream vigorously by hand with a spatula until the mixture lightens and becomes smooth and airy, or use a mixer with the paddle on medium for about 2 minutes. Add the egg and mix until fully incorporated.

Add the dry flour mixture to the butter mixture all at once and gently fold until just combined. Stir in the semisweet chips, mini dark chips, and chopped walnuts, folding until they are distributed evenly through the dough.

Portion the dough using a 2-ounce scoop so each cookie is about 4 ounces (use two scoops per cookie) or divide the dough into six even portions. Press two scoops together and shape into a rough ball. Place the six dough mounds on the prepared sheet about 3 inches apart.

Bake at 375°F for 3 minutes to jump-start rise, then immediately reduce the oven temperature to 350°F and continue baking for 10–12 minutes, until the tops are deeply golden while the centers remain slightly soft.

Remove the baking sheet to a cooling rack and let the cookies rest on the tray for 3 minutes before transferring them to the rack to cool completely.