Preheat the oven to 350 degrees F. Place an oven rack in the middle position. Lightly spray the bottom of 3 8-inch cake pans with baking spray and then cover them with parchment rounds. Set aside.

Whisk together the flour, cornstarch, baking powder, salt and pumpkin pie spice in a medium bowl. In a separate bowl, whisk together the buttermilk, pumpkin puree and vanilla extract.

In a stand mixer with the paddle attachment, cream the butter and sugar together on low speed for 5 minutes. Add the eggs, one at a time, allowing each egg to incorporate before adding the next. Scrape down the sides of the bowl all the way to the bottom. With the mixer still on low speed, swiftly alternate between adding the dry and liquid ingredients over the course of a minute. Scrape down the sides well, making sure to reach the bottom of the bowl. Turn the mixer on medium speed for 30 seconds. Give the batter one final stir by hand to make sure the batter is completely incorporated.

Distribute the batter evenly between the 3 prepared cake pans, using a spatula to even out the tops.

Bake until the a toothpick comes out of the center of each cake clean, approximately 36-40 minutes. Allow the cakes to cool completely in the pan before removing.

Add sugar and water to a medium sized sauce pot. Cover and turn the heat to high. Once the liquid begins to simmer and steam has developed, remove the cover (this helps prevent crystallization). Using a candy thermometer, cook the sugar to the soft boil stage, 235-240 degrees F.

While the sugar is cooking, whisk the eggs on high in a stand mixer with the whisk attachment until a soft peak has formed.

Turn the mixer speed down to medium low and very slowly pour the syrup down the side of the bowl into the egg whites. Don’t pour the hot syrup directly into the meringue or you’ll get scrambled eggs.

Once the syrup is completely incorporated, turn the speed to high and allow the meringue to continue to form a stiff peak while cooling down. Mix on high until the meringue has reached room temperature. To speed up this 15-20 minute process, ice packs can be placed around the bowl.

Once the meringue is room temperature, slowly begin incorporating the soft butter on medium speed. Don’t rush this step. Once all of the butter has been incorporated, turn the mixer speed up to high and slowly add the maple syrup.

Place the first layer of cake on a revolving cake stand and remove the parchment paper. Placing a cardboard round below the cake is optional but will make transporting the cake easier after it is assembled.

Spread approximately one cup of buttercream on the cake and spread it around evenly with an offset spatula. Add more buttercream as needed to reached the desired thickness. Repeat with the second and third layer (don't forget to remove the parchment each time). Use an offset spatula to apply a thin layer of frosting to the side and top of the cake. Chill in the refrigerator for 30 minutes to set this first layer of buttercream. Remove from the refrigerator and cover the entire cake with a final layer of frosting.

Cake is best when served at room temperature.