Heat oven to 375°F and spray two 8 x 12-inch baking pans with cooking spray (if you don't have two 8x12 pans, use one 12 x 16-inch pan, and use 1 cup less dough. Line the pan with parchment, so that each edge of the pan has a 2-inch over-flap. Spray the parchment with nonstick cooking spray.

In the bowl of a stand mixer, cream the butter and sugars together until creamy. Add eggs and vanilla and beat well.

In a medium bowl, whisk together the flour, baking soda, cinnamon, nutmeg and salts. Add the flour mixture to the wet mixture all-at-once and beat only until flour mixture is incorporated.

Add the oats and mix until the oats are incorporated.

Divide the dough in-half and spread half in each pan. Spray a wooden spoon or rubber spatula with cooking spray to prevent the dough from sticking. Work the dough evenly until it reaches across the pans evenly. (If necessary, spray your fingers with cooking spray and spread the dough with your fingers. The dough is somewhat stiff and sticky.)

Bake 12-13 minutes or until the dough is cooked in the middle and is light golden brown. Do not over-bake or the bars will dry out.

Use an electric mixer to beat the softened butter until fluffy. Add the powdered sugar and beat until lighter in color and smooth. Add marshmallow cream, vanilla and salt. Beat until light and fluffy. Set aside until ready to use.

Transfer the cookie bars to a cooling rack to cool 10 minutes. Lift the bars out of the pans, by the parchment paper and set them back on the cooling rack to completely cool.

When bars are cool, spread the Cream Filling over one of the uncut 8 x 12-inch cookies. Remove the parchment paper and place the other 8 x 12-inch cookie over the Cream Filling and press gently down. Place the entire large cookie on a larger baking tray and cover it with plastic wrap and foil. The cookies are easy to cut when frozen. Place in the freezer at least 2 hours or overnight.

Cut the frozen cookie into bars.

Enjoy!