Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper. In a medium bowl, sift together the all-purpose flour, baking powder, salt, and cornstarch. Set aside.

In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer until light and fluffy, about 3-4 minutes. Add eggs one at a time, beating well after each addition. Then mix in the vanilla extract.

Alternate adding the dry ingredients and milk into the butter mixture in three parts, beginning and ending with the dry ingredients. Mix until just combined. Finally, stir in the oil until fully incorporated.

Divide the batter evenly between the prepared pans. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Remove from oven and let cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

For the frosting, beat the butter until creamy. Gradually add the confectioners’ sugar, cocoa powder, and salt, mixing on low speed to combine.

Pour in the melted chocolate, heavy cream, and vanilla extract, then beat on medium-high until light and fluffy. Once the cakes are completely cooled, spread the frosting evenly between the layers and on the top and sides of the cake.