Black Cocoa Sandwich Cookies

Name: Black Cocoa Sandwich Cookies
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: World Cuisine
Ingredients: 226g or 1 cup butter (softened and unsalted ); 200g or 1 cup granulated sugar (200g); 2 teaspoons fine sea salt; 2 teaspoons vanilla extract; 2 large egg whites; 260g or 2 cups all purpose flour; 120g or 1 ¼ cups black cocoa (double dutch cocoa powder or mix of black and dutch); ½ teaspoon baking soda