Preheat your oven to 350°F (175°C). Prepare three 8-inch round cake pans by lining the bottoms with parchment paper and lightly greasing the sides. If you have bake-even strips, wrap them around the pans for more evenly baked cake layers.

In a large mixing bowl, combine the granulated sugar, vegetable oil, eggs, and vanilla extract. Beat on medium speed until the mixture becomes smooth and fully blended, pausing occasionally to scrape down the sides and bottom of the bowl. Add the sour cream and continue mixing until the batter is creamy and well incorporated.

In a separate bowl, sift together the all-purpose flour, baking powder, and salt to evenly distribute the leavening agents and remove any lumps.

Add half of the dry ingredient mixture to the wet ingredients, followed by half of the milk. Mix on low speed just until combined. Repeat with the remaining flour mixture and milk, stirring only until the batter is smooth. Finish by folding the batter with a spatula, scraping the bottom and sides of the bowl to ensure everything is evenly incorporated without overmixing.

Divide the batter evenly among the prepared cake pans, using approximately 1⅔ cups of batter per pan. Smooth the tops with a spatula.

Bake for 20 to 23 minutes, rotating the pans halfway through baking to encourage even browning. The cakes are ready when a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.

Remove the cakes from the oven and let them cool in the pans for several minutes before transferring them to a wire rack. Allow the layers to cool completely before frosting.

Place the cold butter pieces into the bowl of a stand mixer fitted with the paddle attachment. Beat on medium speed for 5 to 7 minutes, scraping the bowl occasionally, until the butter becomes pale, light, and exceptionally fluffy.

Add 3 cups of powdered sugar and mix on low speed until fully incorporated. Pour in the vanilla extract and continue mixing for about 1 minute.

Add another 2 cups of powdered sugar and mix on low speed until combined. Increase the mixer speed to medium-high and whip the frosting for 3 minutes to create a light, airy consistency.

Continue adding the remaining powdered sugar 1 to 2 cups at a time, alternating with small amounts of heavy whipping cream. Mix on low speed after each addition, then increase to medium-high for a couple of minutes to develop a silky, spreadable frosting.

Add a pinch of salt to balance the sweetness. To eliminate excess air bubbles, reduce the mixer to its lowest speed and mix for about 1 minute. Finish by stirring the frosting gently with a spatula to achieve an ultra-smooth texture.

To Assemble the Cake

Once the cake layers have cooled completely, use a cake leveler or serrated knife to trim any domed tops, creating flat, even layers.

Spread a small amount of frosting onto your cake board or serving plate to anchor the cake. Position the first cake layer on top.

Fill a large piping bag fitted with a wide round piping tip—or simply trim the end of the bag—and pipe approximately 2 cups of frosting over the first cake layer. Begin around the outer edge and work toward the center before smoothing the frosting with an offset spatula.

Place the second cake layer on top and repeat the frosting process. Finish by adding the third cake layer.

Pipe frosting into any gaps between the layers, then apply a thin crumb coat over the top and sides of the cake. Smooth the frosting with an angled spatula or cake scraper, removing any excess while leaving a thin, even coating. Discard any crumb-coated frosting that contains cake crumbs rather than returning it to the mixing bowl.

Apply the final layer of buttercream over the entire cake using an offset spatula or a large piping bag fitted with a cake icing tip. Smooth the top and sides with an icing smoother until the finish is clean and even.

Decorate the cake with buttercream flowers and additional piping details as desired. Chill briefly before serving to help the frosting set and maintain crisp decorative edges.