For the pumpkin cakes:Preheat oven to 350 degrees F. Grease a cream canoe pan and set aside.
Combine the flour, sugar, baking powder, salt, and pumpkin pie spice in a medium bowl.
Add in the pumpkin puree, oil, vanilla extract, and egg yolks and whisk until fully combined.In a stand mixer or separate bowl, combine the egg whites and cream of tartar, and whip to stiff peaks.Fold in about a third of the egg whites into the cake batter to lighten. Fold in the rest of the egg whites until incorporated, trying not to deflate them too much.Fill the canoe pan cavities about two-thirds full.
Bake for about 9-11 minutes until the cakes are puffed up and fully set.
Combine flour and milk in a small saucepan and cook over medium heat until it thickens into a paste.
Remove from heat and scrape into a bowl. Press a piece of plastic wrap over the top and let finish cooling.
Combine shortening, butter, sugar, and salt in a stand mixer and beat until combined.
Combine all ingredients in bowl of a stand mixer and beat until smooth and combined. If it is a little too runny, you can add more confectioners' sugar; if it's too thick, add some more cream.To assemble:Fill a pastry bag fitted with a round tip with the filling. Poke tip into the underside of a cake and squeeze filling into cake. Repeat along length of cake (three spots total should be fine).
