In a small mixing bowl, whisk together flour, cornstarch, cinnamon, baking soda, and salt. Set aside.

In a large mixing bowl, cream butter with an electric mixer until light and fluffy, about one minute. Beat in dark brown sugar. Add eggs one at a time, followed by vanilla. With the mixer on low, add the flour mixture in two installments, until completely combined.

In a food processor process the oats and raisins for one minute.

Mix in quick oats and raisins

Cover dough with plastic wrap and refrigerate for at least one hour, or up to three days.

Preheat oven to 400F. Line two sheet pans with parchment paper.

Scoop cookie dough in ½ tablespoon (1 ½ teaspoon) increments. Roll into balls and set them at least 2 ½ inches apart on prepared pans. Flatten balls to 2 ½ inches.

Bake for 4- 5 minutes on the middle rack until the edges look dry.

Let cool on the pans for five minutes before removing to a rack to cool completely. Repeat rolling and baking until you have used all the dough.

To make the filling, cream the marshmallow fluff and shortening with an electric mixer until light and fluffy. Beat in confectioner’s sugar and salt. Add in vanilla and hot water and mix on high until the filling is very fluffy.

To assemble a sandwich cookie by piping, apply filling by pipe about a teaspoon onto the middle of the underside of one cookie. Top with a second plain cookie, with the underside filling-side-in. Repeat until all cookies have been used.

To assemble a sandwich cookie by spreading, use an offset frosting knife to spread ½-1 teaspoon on the underside of one cookie. Top with a second plain cookie, with the underside filling-side-in. Repeat until all cookies have been used.

Cookies will keep very well in an airtight container at room temperature for up to a week.