Preheat oven to 350 degrees. In a mixing bowl whisk together flour, cornstarch, cinnamon, nutmeg, baking powder, baking soda and salt for 30 seconds, set aside.
In the bowl on an electric stand mixer fitted with the paddle attachment, whip together butter, 1 Tbsp of the vegetable oil, 3/4 cup granulated sugar, and light-brown sugar on medium-high speed until pale and fluffy.
Blend in remaining 2 Tbsp vegetable oil. Mix in eggs one at a time blending until combined after each addition and adding in vanilla with second egg.
Blend together milk and buttermilk in liquid measuring cup or bowl. With mixer set on low speed, add in 1/3 of the dry ingredients in three separate batches alternating with 1/2 of the milk mixture, beginning and ending with flour and mixing just until combined after each addition.
Divide batter among paper lined muffin cups, adding about 1/4 cup batter to each cup and filling 2/3 full. Bake in preheated oven 17 - 20 minutes, until toothpick inserted into center comes out clean.
Remove from oven and allow to cool in muffin tin several minutes before transferring to a wire rack.
Allow to cool completely then frost with vanilla buttercream frosting, then in a small bowl, whisk together remaining 2 tsp granulated sugar and 1/2 tsp cinnamon. Dust tops of cupcakes lightly with cinnamon sugar mixture.
For the frosting: In the bowl of an electric stand mixer, using the paddle attachment whip butter on medium high speed until nearly white and very fluffy, about 5 - 8 minutes.
Blend in vanilla extract. Add in powdered sugar and heavy cream and mix on low speed until combined.
Increase speed to medium and beat until very light and fluffy, about 5 - 6 minutes, frequently scraping down the sides and bottom of the bowl. Spread over cooled cupcakes.