Preheat oven to 350.
Using your whisk attachment, mix eggs on medium speed in your stand mixer (I mix mine on "6") for 2 minutes. They should be creamy and have a thickness to them.
While the eggs are mixing, prepare some of the dry ingredients: in a separate bowl, whisk together flour, baking soda, baking powder, and salt. Set aside.
Turn mixer to low, add sugar to eggs, and mix on medium speed again for 2 minutes.
Scrape sides of bowl & add softened butter & vanilla. Mix on medium speed for one more minute.
Turn mixer to low and gradually add in flour mixture. Mix until just incorporated.
Scrape sides of bowl, turn mixer back onto low, and gradually add in buttermilk & oil. Mix until everything is fully incorporated, and then stir in cinnamon. Set aside while you make the cinnamon sugar crumble.
In a smaller mixing bowl, add 1.5 tbsp butter, brown sugar, and 3/4 tsp cinnamon. Use a fork to mash the butter and mix the crumble together. Set aside.
Pour a little bit of batter into 12 cupcake wrappers in a cupcake pan - maybe about 1-2 tbsp of batter in each
Sprinkle a little bit of the crumble mixture on top of each, then add more batter on top - fill the cupcake wrappers about 3/4 of the way to the top. Add more crumble mixture on top of each.
Place cupcake pan in oven and bake at 350 for 17-18 minutes. Check with a toothpick - if doesn't come out clean, try a couple more minutes. (Mine are always perfect right at 17 minutes.)
Remove from oven and allow cupcakes to cool a bit before transferring them to a wire cooling rack. (The cupcakes are somewhat delicate while they're still warm, so be careful as you transfer them from the pan to the cooling rack.)
Repeat with remaining cupcake batter & cupcake crumbles. (This recipe makes about 30 cupcakes, so you'll have your pan in the oven 3x.)
While cupcakes are cooling, prepare the cream cheese frosting.
Using the paddle attachment, mix cream cheese on medium speed for 1 minute. Add butter & powdered sugar and cream together for 2 minutes on medium speed. Mix in vanilla, cinnamon, and pinch of salt. Scrape the sides of the bowl, mix again if necessary, and then transfer icing to a piping bag to frost cooled cupcakes.
Sprinkle a little bit of cinnamon on top for garnish, and enjoy!