Preheat oven to 350° and prepare a 11" x 16" rimmed baking sheet.
To start, place butter + water + candy corn in a medium saucepan over medium to medium high flame. Stir continuously until the butter and candy corn have melted and the liquid is at a low simmer.
Remove the candy corn mixture from the flame and add sour cream, mixing thoroughly.
In a medium bowl, combine flour + baking soda + sea salt. Give these dry ingredients a stir to evenly combine.
Whisk two eggs.
Add half of the dry ingredients to the candy corn mixture and stir to incorporate. Add half of the whisked egg mixture, continuing to stir. Add the remaining flour and stir. Finish off by adding the remaining egg, stirring to combine until the cake batter is smooth and free of lumps.
Pour the candy corn cake batter into a prepared 11" x 16" rimmed sheet pan and bake in a preheated 350° oven for 15-18 minutes or until the center is set.
While the cake is baking, prepare the stove-top buttercream by adding butter and milk to a medium saucepan. Over medium flame, melt butter and bring mixture to a low simmer. Remove the milk + butter mixture from the heat and add powdered sugar.
Stir vigorously to remove lumps. Depending on the amount of evaporation that occurred while melting the butter, you may need to add additional milk or powdered sugar to reach your desired consistency.
Once the cake has finished baking, allow it to cool for 10 to 15 minutes before pouring the warm stove-top buttercream over the cake. Be careful while pouring the buttercream as you'll likely have a little more icing than needed to cover the cake. If you pour it too fast, it may overflow!
Immediately after topping this candy corn cake with buttercream, top with seasonal sprinkles.