For the cake:

1. Preheat the oven to 350°F. Prepare three 8-inch round baking pans, line the bottom with parchment paper, and grease the sides. Prepare with bake-even strips if desired.
2. In a large mixing bowl, combine the sugar, vegetable oil, eggs, and vanilla extract. Beat on medium speed until well incorporated. Scrape down the bowl as needed. Next mix in the sour cream and beat until well combined.
3. In a separate bowl combine the remaining dry ingredients and sift together the flour, baking powder, and salt. Add half the dry ingredients, and half the milk, and beat on low speed just until the flour starts to incorporate. Repeat until all ingredients are added and combined. Use a spatula to scrape down the sides of the bowl and stir the batter from the bottom to the top to ensure it’s well mixed.
4. Divide the batter evenly between the three pans, about 1 2/3 cups of batter per pan. Bake at 350°F for 20-23 minutes. Rotate your pans in the oven halfway through baking (around 10 minutes). Test the cake for doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. Remove from the oven and allow to cool completely.

For the frosting

1. Cut butter into pieces. Using the paddle attachment, whip the butter for 5-7 minutes, scraping down the bowl occasionally. Beat until the butter is light, fluffy, and whipped to a spreadable consistency. 
2. Add 3 cups of powdered sugar and mix on low speed until incorporated into butter then add the vanilla extract and mix to combine. Beat for about 60 seconds.
3. Next, add an additional 2 cups of powdered sugar and beat on low speed until sugar is incorporated. Increase speed to medium-high and beat for 3 minutes.
4. Add the remaining powdered sugar 1-2 cups at a time alternating with the heavy whipping cream. Beat on low speed until ingredients start to mix together. Increase speed to medium-high and beat for another couple of minutes to whip additional air into the frosting. 
5. To remove the air bubbles, reduce the mixer to the lowest speed for about 1 minute, then mix by hand using a spatula to remove any additional air bubbles. 

To assemble the cake

1. Use a cake leveler to cut the domes off the top of the cake. Place a dollop of frosting on your cake board and place the bottom layer of cake on top of that.
2. Place about 2 cups of frosting in a large piping bag fitted with a large open round tip, or simply cut off the end of the piping bag. Pipe a layer of frosting on top of the cake, starting at the outside edge and working your way into the middle. Spread evenly with an offset spatula. Repeat with the second layer of cake and then place the third layer on top.
3. Use a piping bag to fill in any gaps between layers and to create a crumb coat. Using an angled spatula or icing smoother, wipe away the excess frosting, leaving just enough to coat the outside of the cake. Do not mix the crumb-coat frosting back into the mixing bowl if it has cake debris in it.
4. Proceed to frost the top and sides of the cake using your angled spatula or a large piping bag fitted with a large cake icer tip. Once the sides of the cake are covered, use an icing smoother to remove any excess frosting.