Heat oven to 375°F and arrange rack in the middle of the oven.
Combine 2/3 cup mashed avocado and tablespoon of vanilla extract in a bowl.
Combine 2 3/4 cup flour, 2 teaspoons baking soda, and 1 teaspoon salt in a medium bowl. Whisk to break up any lumps and combine.
Transfer avocado mixture, 1/2 cup coconut oil, 3/4 cup sugar and 3/4 cup brown sugar in the bowl of a stand mixer fitted with the paddle attachment. Mix on low until incorporated then increase speed about 1 setting and beat until sugar is mostly dissolved and mixture is creamy.
Add flour mixture and mix on low until combined. Add 12 ounces chocolate chips and 1 cup walnuts and mix on low until evenly incorporated throughout the dough.
Scoop 2-3 tablespoon-sized pieces of dough and place on a parchment-lined baking sheet about 1-inch apart (they don't spread much). Flatten balls slightly and sprinkle a little sea salt on top of each cookie.
Bake in the top of the oven for 12-15 minutes, or until browned around the edges, but still soft in the middle.
Let cool on baking sheet at least 10 minutes. Repeat with remaining dough.