Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease and flour the two 9-inch round cake pans.
Mix Dry Ingredients: In a mixing bowl, whisk together the flour, sugar, baking powder, and salt.
Blend Wet Ingredients: In another bowl, cream together the softened butter and vegetable oil. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Combine Wet and Dry: Gradually add the dry ingredients to the wet mixture, alternating with the milk. Begin and end with the dry ingredients. Mix until just combined; do not overmix.
Divide and Bake: Pour the batter evenly into the prepared pans. Bake for 25 to 30 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then remove and cool completely on a rack.
Prepare Chocolate Frosting: In a mixing bowl, sift the cocoa powder to remove lumps. Cream the softened butter, then add the sifted cocoa, confectioners' sugar, milk, vanilla extract, and a pinch of salt. Beat until smooth and creamy.
Frost the Cake: Once the cakes are completely cool, place one layer on a serving plate. Spread a layer of chocolate frosting on top. Place the second cake layer and frost the top and sides evenly.
Decorate (Optional): If desired, you can decorate the cake with chocolate shavings, sprinkles, or more cocoa powder.