Make the cookies
Preheat the oven to 425 degrees F. Lightly grease a baking sheet and set aside (or use parchment).

Cream the butter, sugar, and eggs in a large bowl.

In a separate bowl, sift together the salt, flour, and baking powder. Add to the creamed mixture. Add the cinnamon and oats. Mix well.

In a small dish, add the baking soda to the boiling water, and then stir the mixture into the rest of the batter. Mix well.

Drop by the tablespoon onto the baking sheets about 2 inches apart and bake until the cookies are firm and just starting to turn golden around the edges, 10 to 15 minutes. Cool the cookies on a wire rack or a plate.

Make the filling
In a small measuring cup, sprinkle the gelatin over 1/3 cup cold water. Let stand, stirring once or twice until the gelatin softens, about 5 minutes.

In a heavy 2-quart saucepan over medium-high heat, stir together the granulated sugar, corn syrup, and 1/4 cup hot water until well blended. Continuing to stir, raise the heat to high and bring to a full boil. Continue boiling for 30 seconds. Immediately remove from the heat. Stir in the gelatin mixture until it dissolves completely.

Transfer the mixture to a large bowl. Add the vanilla.

With a mixer on medium speed, beat for 20 seconds. Gradually raise the speed to high and beat until the mixture is stiffened, white, very fluffy, and cooled to barely warm, about 5 minutes.

Reduce the mixer speed to low and gradually add the powdered sugar.

Add the shortening and beat until completely smooth.

The filling will set up as it sits, so wait until it is thick enough to spread or pipe before filling the cookies.