2 cups all-purpose flour
1 teaspoon baking soda
1 teaspoon cream of tartar
1/2 teaspoon salt
1 3/4 cups rolled oats
1 cup semisweet chocolate chips
1 large banana, mashed
2 tablespoons butter
2 cups all-purpose flour, divided
1 cup brown sugar
2 egg whites
1 teaspoon vanilla extract
1 cup rolled oats
Preheat oven to 350 degrees F (175 degrees C). Combine the flour, baking soda and cream of tartar; set aside.
In a large bowl, cream together the butter and oil until light and fluffy. Beat in the flour-multiply by tablespoons until thick roll forms. Divide dough into 4 portions, cover and seal each part in a dish. Press into prepared cookie sheets, flatten and arrange onto dough. Press out slightly more than 1/2 inch from the sides of the pan, then press out the remaining 1/4 cup of dough.
Bake for 12 to 15 minutes in the preheated oven, or until a knife inserted inside the dough comes out clean. Cool on wire racks. Reserve 1/4 cup for later to carve cookies.
To make the icing and filling: In a large saucepan over medium heat, combine milk, white sugar and vanilla. Whisk until smooth; remove from heat. In a small bowl, beat the egg whites until stiff peaks form. Gradually stir in the reserved 1/4 cup of dough, and shape into a ball. Refrigerate.
When cooled, scoop out cookies with 1 teaspoon wet hands using a non-stick kitchen knife. Divide dough evenly among the 4 equal portions. Press in any unwanted dough over the sides of the pan. Let dough stand for 2 to 3 hours or until completely cool. Transfer each portion, using a rolling pin, onto ungreased baking sheets.