Vegan Double Chocolate Cookies

Name: Vegan Double Chocolate Cookies
Ingredients: 10 tablespoons (1 1/4 sticks) vegan stick butter; 3 ounces vegan dark chocolate (70 to 75% cocoa); 1/2 cup toasted walnuts (optional); 1 2/3 cups all-purpose flour; 1/3 cup Dutch-processed unsweetened cocoa powder; 1 1/2 teaspoons baking powder; 1/4 teaspoon instant espresso powder (optional); 1/2 teaspoon kosher salt; 2/3 cup packed light brown sugar; 1/3 cup granulated sugar; 1/4 cup unsweetened non-dairy milk, such as almond, oat, or soy; 1 1/2 teaspoons vanilla extract; 2/3 cup semi-sweet chocolate chips; 1/4 cup cocoa nibs (optional); Flaky salt, for sprinkling