Preheat the oven to 350°F (175°C). Grease two 8-inch round pans and line with parchment. Whisk flour, baking powder, baking soda, and salt in a large bowl. In a separate bowl, beat oil, sugar, eggs, buttermilk, and 2 tsp vanilla until smooth. Add wet to dry and stir just until combined.

Divide batter between pans. Bake 28 to 32 minutes until a toothpick inserted in the center comes out clean. Cool in pans 10 minutes, then turn out onto a wire rack to cool completely.

For the frosting, beat the butter until creamy, about 2 minutes. Sift in powdered sugar and cocoa, mix on low, then add 4 tbsp cream, 1 ½ tsp vanilla, and salt. Beat 2 minutes until glossy, adding up to 1 more tbsp cream if needed.

Place one cake layer on a plate, spread with 1 cup frosting, top with the second layer. Frost the top and sides with the remaining frosting. Chill 15 minutes if you want cleaner slices, then slice and serve.