Lightweight cocoa cookies

Ingredients

• 5 tablespoons butter
• 1/2 cup cocoa powder
• 1 cup all purpose unbleached flower
• 1/4 teaspoon baking soda
• 1/4 teaspoon salt
• 2/3 cup sugar
• 1/3 cup dark brown sugar
• 1/3 cup yogurt
• 1 teaspoon vanilla extract
• 2 teaspoons to 1 tablespoon of water or milk