1 (15-ounce) can (425 grams) pumpkin1½ cups (330 grams) firmly packed light brown sugar1 cup (224 grams) vegetable oil3 large eggs (150 grams)3 cups (375 grams) all-purpose flour1½ tablespoons (9 grams) pumpkin spice1 tablespoon (15 grams) baking powder1 teaspoon (5 grams) baking soda1 teaspoon (3 grams) kosher salt1 cup (113 grams) chopped walnutsPumpkin Spice Frosting (recipe follows)
Preheat oven to 350°F (180°C). Line bottoms of 3 (8-inch) round cake pans with parchment paper; lightly butter and flour sides of pans.
In the bowl of a stand mixer fitted with the paddle attachment, beat pumpkin, brown sugar, oil, and eggs at medium speed until well combined, about 1 minute.
In a medium bowl, whisk together flour, pumpkin spice, baking powder, baking soda, and salt. With mixer on low speed, gradually add flour mixture to pumpkin mixture, beating until well combined. Fold in walnuts. Divide batter among prepared pans.
Bake until a wooden pick inserted in center comes out clean, 28 to 30 minutes. Let cool in pans for 10 minutes. Remove from pans, and let cool completely on wire racks. Spread 1 cup Pumpkin Spice Frosting between each layer and on top of cake. Lightly spread Pumpkin Spice Frosting on sides of cake. Refrigerate for 30 minutes.
Place half of remaining Pumpkin Spice Frosting in a pastry bag fitted with a 4B star tip. Place remaining Pumpkin Spice Frosting in a pastry bag fitted with a Wilton 2A round tip. Pipe Pumpkin Spice Frosting on top of cake as desired. Serve immediately. Cover and refrigerate for up to 3 days.