Preheat the oven to 180° C. Place baking paper on two rectangular baking trays of approximately 45cm x 33cm.
In a medium bowl, sift the flour with the cinnamon, baking soda and salt.
In another large bowl, beat the butter with the two types of sugar with an electric mixer or balloon whisk for 2-3 minutes until the mixture is light and fluffy.
Add the eggs plus the molasses and vanilla extract. Beat for a further 1 minute until just combined.
Add the flour mixture and beat just 30 seconds until smooth.
Finally, add the oat flakes and beat until well combined and smooth.
Spoon out portions of the dough and form balls of approximately 3-4 cm in diameter (can be done with the measure of 1 1/2 measuring spoons). There should be 30 balls of dough.
Place the dough balls in the moulds 5 cm apart so that they do not stick together when baking.
Bake at 180° C for 10-12 minutes until lightly browned and the centre looks a little raw (this is because they should be very soft).
Remove the biscuits from the oven and leave to cool completely on a wire rack before filling.
For the filling and assembly:
Place the butter with the shortening in a large bowl and beat with an electric mixer or balloon whisk for 2-3 minutes until very smooth.
Add the icing sugar with the vanilla extract, salt and beat for a further 1 minute until fully combined.
Add the marshmallow creme and beat for 2 minutes until very creamy and smooth.
Take half of the biscuits and place them base side up to spread enough filling using a spoon. Then take the other half of the biscuits and place them on top of the biscuits with the filling to form biscuit sandwiches or whoopie pies.
Let the whoopie pies rest for at least 15 minutes before eating to firm up.