Preheat the oven to 350°F. Lightly grease three 8-inch round pans with baking spray or coat them with butter and flour. Line the bottoms with parchment paper, and wrap the pans with soaked fabric baking strips if you have them.

In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt, then set aside.

In the bowl of a stand mixer using the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 3 minutes. (You can also use a large bowl and an electric hand mixer.)

Add the oil and vanilla, mix to combine then scrape the bowl down. Add the eggs one at a time, mixing until incorporated before adding the next. Scrape the bowl down once more and mix again.

With the mixer running on low speed, add flour mixture in three batches alternating with the buttermilk. Scrape down the bowl and mix again if needed. Divide the batter evenly among the three prepared pans. Smooth it into an even layer in each pan.

Bake at 350°F for about 25 to 30 minutes or until the centers are set and springy when gently pressed. Allow the layers to cool in pans for 10 minutes then invert them onto a wire rack, remove the parchment paper, and cool completely.

Place ¼ cup (56g) of the butter in the microwave-safe bowl and melt in 15 second intervals. Mix in the cocoa powder and set aside.

Combine the semisweet chocolate with the milk in another microwave-safe bowl, and heat in 30-second increments, stirring between each until melted and smooth. Set the ganache aside to cool slightly.

In the bowl of a stand mixer using a paddle attachment (or a large bowl if using an electric hand mixer), beat the remaining softened butter on medium speed until creamy, about 1 minute.

Pour in the cocoa mixture, mix to combine, then stop to scrape the bowl down and mix again. While mixing on low speed, gradually add the powdered sugar. Once combined, pour in the cooled ganache and beat until combined and the frosting is fluffy and spreadable. Stop and scrape the bowl down as needed. Your frosting should be silky, soft and luscious. If it feels stiff, add a bit more milk, a teaspoon at a time.

Place one cake layer on a cake stand or plate, spread 1 cup (238g) of the buttercream on top of the layer. Add the next layer on top and repeat with more frosting. Add the final layer, bottom side up so the top of the cake is flat.

Cover the cake with the rest of the buttercream, then use a spatula to swirl the buttercream around for a rustic effect. Chill the cake for 30 minutes before slicing to set the frosting.