Preheat the oven to 375°F. Grease a 14-inch rimmed pizza pan and set it aside.

In a large bowl or the bowl of a stand mixer, beat the softened butter, sugar, and brown sugar together until light and creamy, scraping the bowl as needed.

Add the vanilla extract and the eggs (add eggs one at a time if you prefer), and beat until the mixture is smooth and homogenous.

Add the all-purpose flour, salt, and baking soda to the bowl. Mix on low speed or stir by hand just until the dry ingredients are incorporated and no large streaks of flour remain—do not overmix.

In a small bowl, combine the semisweet chocolate chips, white chocolate chips, and chopped walnuts. Reserve about 1/4 of this mixture for the top of the cookie, and fold the remaining 3/4 gently into the dough until evenly distributed.

Spread the cookie dough into an even layer on the prepared pan (use a spatula or lightly damp hands to smooth it). Sprinkle the reserved chocolate chips and walnuts evenly over the top and press them lightly into the surface.

Bake on the middle oven rack for 18–20 minutes, or until the edges are golden brown and the center is set.

Remove the pan from the oven and let the cookie cool on the pan for about 10–15 minutes before slicing and serving.