Preheat your oven to 350°F (175°C). Grease and flour two 9-inch cake pans or line them with parchment paper.

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

In a large mixing bowl, use an electric mixer to cream the softened butter and granulated sugar together on medium speed for about 3-4 minutes, or until light and fluffy.

Add the eggs, one at a time, beating well after each addition. Make sure each egg is fully incorporated before adding the next one.

In a separate bowl, combine the buttermilk, canola oil, and vanilla extract. Slowly add this mixture to the butter and sugar mixture, alternating with the dry ingredients, starting and ending with the dry ingredients. Mix until just combined.

Divide the batter evenly between the prepared cake pans, smoothing the tops with a spatula.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center of the cakes comes out clean.

Let the cakes cool in the pans for about 10 minutes, then transfer them to a wire rack to cool completely.

While the cakes are cooling, prepare the chocolate buttercream frosting. In a medium bowl, beat the softened butter until creamy. Gradually add the powdered sugar and cocoa powder, then beat in the salt, vanilla extract, and milk or cream. Beat until the frosting is smooth and fluffy.

Once the cakes are completely cooled, spread a layer of chocolate buttercream frosting on top of one cake layer. Place the second cake layer on top and frost the top and sides of the cake with the remaining buttercream. Smooth it out with a spatula or cake knife for a beautiful finish.