1. Preheat the Oven: Preheat the oven to 350°F (180°C). For convection ovens, set to 325°F (160°C). Line two 8-inch round pans with parchment paper and lightly grease the sides with flour or cake pan release.

2. Prepare the Dry Ingredients: Sift together flour, baking powder, baking soda, and salt in a medium bowl. Set aside.

3. Cream the Butter and Sugar: In a large bowl, beat butter and sugar until light and fluffy, about 2-3 minutes. Scrape down the bowl.

4. Add Oil, Vanilla, and Eggs: Mix in oil and vanilla. Add eggs one at a time, beating after each addition and scraping the bowl.

5. Add Dry and Wet Ingredients: Add 1/3 of the flour mixture, then the sour cream. Add another 1/3 of the flour mixture, then milk, followed by the remaining flour mixture. Mix until just combined.

6. Prepare the Pans: Divide the batter evenly between the pans. Smooth the tops and drop the pans to remove air bubbles.

7. Bake: Bake for about 30 minutes, or until a toothpick comes out clean. Let the cakes cool for 10-15 minutes before transferring to a wire rack.

For the Frosting:

1. Beat the Butter: Beat butter until smooth and fluffy.

2. Add Powdered Sugar and Cocoa: With the mixer on low, beat in 2 cups powdered sugar, sifted cocoa, vanilla extract, and salt.

3. Add Cream and Remaining Sugar: Gradually add the remaining powdered sugar, alternating with 1 tablespoon of cream or milk until the desired consistency is reached.

For Assembling:

1. Prepare the Cake Layers: Once the cakes are cool, level the tops if domed.

2. Layer the Cake: Place one layer of cake on a cake stand or plate, frosting the top with 1/3 of the frosting.

3. Add the Second Layer: Add the second cake layer, crumb coat the sides with a thin layer of frosting, and let it firm up. Apply a final thick layer of frosting.

4. Decorate: Use remaining frosting to smooth over the cake. Decorate with sprinkles, candles, or any other toppings.