Prepare oven and cake pans: Preheat oven to 350°F. Coat 2 (8-inch) cake pans with cooking spray; line bottoms of pans with parchment paper and coat paper with cooking spray.
Make cake batter: Whisk together flour, cocoa, baking soda, and salt in a medium bowl. Place butter in a large bowl; beat with an electric mixer at medium speed until smooth and fluffy, 2 to 3 minutes. Add sugar, oil, and vanilla; beat until well combined and creamy, about 2 minutes. Beat in eggs, 1 at a time, beating well after each addition. With mixer on low speed, gradually add flour mixture and buttermilk alternately, beginning and ending with flour mixture.
Add batter to pans and bake: Divide batter evenly between prepared pans. Bake at 350°F until a wooden pick inserted in center comes out clean, 30 to 35 minutes. Cool cake in pans on a wire rack for 10 minutes. Invert cakes onto rack and cool completely.
Assemble cake: Once cooled, assemble cake and frost layers with Ermine Frosting.
