Step 1

Preheat the oven to 350 degrees F.

Step 2

Use the extra butter to coat a 15 1/2x10 1/2-inch deep-sided jelly roll pan, then use the extra flour to coat the greased bottom and sides. Shake the pan and dump out the excess flour.

Step 3

In a medium bowl, add the remaining 3 cups of flour, 1 1/2 tablespoons of the pumpkin pie spice, the baking powder, the baking soda, and 1 1/2 teaspoons of the salt and stir to combine.

Step 4

In the bowl of a stand mixer fitted with the whisk attachment, add 1 1/2 cups of the butter, the granulated sugar, and the brown sugar, creaming together by starting on low speed and gradually increasing to high speed, until it is light, fluffy, about 4-5 minutes.

Step 5

Scrape the sides of the bowl with a spatula.

Step 6

Add the eggs to the butter mixture, one at a time, and beat after each addition.

Step 7

Scrape down the sides of the bowl and reduce the mixer speed to low.

Step 8

Alternate adding the flour mixture and the pumpkin purée to the butter mixture until both are combined and the batter is smooth.

Step 9

Scoop the batter into the prepared jelly roll pan and evenly smooth it out.

Step 10

Bake the cake until the center is puffy and a toothpick inserted into the center comes out clean, about 45-50 minutes. Do not open the oven until the end of the baking time.

Step 11

Transfer the jelly roll pan to a wire rack and let the cake cool completely.

Step 12

Before serving, in the bowl of a stand mixer fitted with the whisk attachment, add the remaining 1 cup of the butter and whip on high speed until it is very fluffy and soft.

Step 13

Reduce the speed to medium-low and mix the buttermilk, the vanilla, and the remaining salt into the whipped butter.

Step 14

Slowly add 5 cups of the powdered sugar, 1 cup at a time, to the frosting mixture and mix after each addition until it is light and smooth. If it isn't thick enough, add 1/2-1 cup of the extra powdered sugar and whip until it is smooth.

Step 15

Spread the frosting over the cooled cake and sprinkle with the extra pumpkin pie spice.

Step 16

Slice and serve.