Preheat the oven to 350°. For the cake, butter and flour two 9-inch cake pans; tap out excess flour. Line the bottom of each pan with a parchment paper round. In a large bowl, whisk 2 3/4 cups flour, the baking powder, allspice and salt. In a small bowl, mix the buttermilk and vanilla. Using an electric mixer on medium-high speed, beat the butter and oil until fluffy, about 3 minutes. With the mixer running, gradually add the sugar. Beat on medium speed until light and fluffy, about 2 minutes. Reduce the speed to low and add the eggs 1 at a time, beating to blend between additions. Alternately mix in the dry ingredients and the buttermilk mixture in three additions, starting with the dry ingredients and beating to blend between additions. Scrape down the sides of the bowl; fold in the sprinkles. Divide the batter between the pans. Bake until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Let cool in the pans on a rack for 10 minutes. Invert onto the rack. Peel off the parchment; let cool.
For the frosting, in a medium bowl, using an electric mixer on medium speed, beat the butter, cream cheese, confectioners’ sugar, vanilla and salt until smooth, about 3 minutes. Transfer half of the frosting to a small bowl. Coarsely chop half of the candy corn and mix into the frosting in the small bowl. Place one cake round on a cake plate; top with the candy corn frosting, spreading to the edges. Top with the second cake round; spread the remaining frosting to the edges. Sprinkle with the remaining candy corn.
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