For the cake:
Preheat oven to 350F.
Have 1 rack one-third from the bottom of the oven and a second rack two-thirds from the bottom.
Line three 9-inch cake pan (please note I used four 6-inch pans for effect for the blog) with parchment paper rounds and spray with baking spray or butter and flour.
Over medium heat, using a medium saucepan, add the buttermilk, milk, and candy corn and bring to a boil.
Once it reaches a boil, remove from heat.
Strain out the candy corn (discard any extra) and set aside.
In a medium bowl, combine flour, baking soda, salt, and baking powder.
Using a stand mixer with the paddle attachment, beat butter and sugar on medium speed until light and fluffy, about 2 minutes.
Scrape down sides of bowl and add egg whites and vanilla.
Beat on medium speed for 1 minute.
Add 1/3 of flour mixture and beat on medium speed until incorporated.
Pour half of candy corn buttermilk mixture and beat on medium until mixed in.
Scrape down sides of bowl and repeat adding flour mixture and candy corn buttermilk mixture, finishing with remaining 1/3 of flour.
Batter will be thick and glossy.
Pour batter evenly into prepared pans, I like to use an ice cream scoop because the batter is so thick.
Set two cake pans on 1 oven rack and the third pan on the second rack, staggering them so cakes are not directly over each other.
Bake 25 to 35 minutes, until the tops are flat and browned.
Cool cakes in pans for 10 minutes.
Invert onto racks to cool. (Cakes can be wrapped in plastic wrap and frozen for up to 3 weeks.)
For the Simple Syrup:
In a medium saucepan heat together the candy corn and water until all the candy corn has dissolved into the water.
Brush the syrup onto each layer of the cake.
Set aside until ready to frost.
For the frosting:
lPace the egg whites and sugar in the metal bowl of a stand mixer and set over a small pot of simmering water (the bowl should not touch the water).
Heat, stirring occasionally, until mixture is 160F or hot to the touch.
Transfer bowl to stand mixer and beat on high speed with a whisk attachment to medium stiff peaks, about 8 minutes.
Reduce mixer speed to medium low and add butter a tablespoon at a time, letting it incorporate into the meringue for few moments before adding more.
Add one TBSP candy corn syrup, melted white chocolate, and salt and beat on high for 1-2 minutes until smooth and creamy.
Assemble the Cake:
Set one cake layer on turntable cake stand.
Cover with 3/4 cup buttercream.
Top with another cake layer.
Repeat till cake is assembled.
Use an offset spatula to frost cake with buttercream.
If desired, first make a thin coat to seal in crumbs and chill for 30 minutes.
Cover completely with another layer of frosting.
Pipe some decorative swirls on top of cake and top with candy corn and other Halloween decor.
Chill cake for 30 minutes.