Step 1

For the cake: Position an oven rack in the middle of the oven and preheat to 350 degrees F. Lightly coat one 9-inch round cake pan and one 9-inch square cake pan with nonstick cooking spray. Line the bottom of each with parchment paper; lightly coat the paper with nonstick spray.

Step 2

Whisk together the flour, cornstarch, baking powder, salt and baking soda in a large bowl until combined. Beat the butter, granulated sugar, oil and vanilla with an electric mixer on medium-high speed until light and fluffy, 3 to 4 minutes. Add the eggs and yolks one at a time, beating to blend between additions and occasionally scraping down the sides and bottom of the bowl until the batter is completely combined, smooth and creamy. With the mixer on low speed, add the dry ingredients in 3 additions, alternating with the buttermilk, beginning and ending with the dry ingredients. Scrape the batter evenly into the prepared pans. Smooth the tops. Bake, rotating the pans halfway through, until the cakes are a light golden brown and a tester inserted into the centers comes out clean, 35 to 45 minutes. Transfer the pans to a wire rack and let the cakes cool for 30 minutes. Run a knife around the sides of the cakes and invert onto the wire rack. Remove the parchment and let the cakes cool completely.

Step 3

For the frosting: Meanwhile, beat the butter in a stand mixer on medium-high speed until very creamy, about 2 minutes. Scrape down the sides of the bowl with a rubber spatula. Mix the milk, vanilla and salt in a liquid measuring cup, then add it to the butter. Mix slowly on low speed until combined. Increase the speed to medium and beat until very smooth. Add half of the confectioners’ sugar and beat on low speed until just combined. Scrape the bowl again with a rubber spatula. Add the remaining confectioners’ sugar and beat on low speed until combined and smooth, 2 to 3 minutes. Scrape down the sides of the bowl, then mix on medium-low speed until fluffy and well combined, about 5 minutes. Transfer 1/2 cup of the frosting to a small bowl and mix in black gel food coloring until very pigmented. Set both bowls of frosting aside at room temperature until ready to assemble the cake.

Step 4

For assembling: Once the cakes have cooled completely, place the square cake, domed-side down, in the center of a large cutting board or platter (this will be the body of the ghost). Place the round cake domed-side down and cut in half with a sharp knife into 2 half-moons. Place one half-moon above the square cake with the cut-side adjoining the body (this will be the head of the ghost). Repeat with the other half-moon below the body (this will be the bottom of the ghost from which two arms will be cut). Next, cut a scalloped edge of 3 curves, each about 2 inches wide, into the bottom of the ghost, keeping the cake scrap intact as one piece. Then cut two oval-shaped arms from that cake scrap. Place one arm on each side of the ghost’s body. There should be minimal cake scraps left; enjoy or save for another use.

Step 5

Use a little white frosting to help adhere the head, bottom and arms to the body. Frost the top and sides of the cake with the remaining white frosting, smoothing out the top with an offset spatula. Transfer the black frosting to a piping bag fitted with a small round pastry tip; pipe two eyes and a mouth onto the ghost. Serve right away or cover and refrigerate for up to 3 days.