Step 1: Prepare the Cake Batter

1. Preheat oven to 350°F (175°C). Grease and line three 8-inch cake pans with parchment paper.
2. In a large bowl, sift together cake flour, baking powder, baking soda, and salt. Set aside.
3. In a separate bowl, whisk together sour cream, key lime juice, and zest. Set aside.
4. Using a stand mixer, cream butter and sugar on medium speed until light and fluffy (about 5 minutes).
5. Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
6. Gradually add the dry ingredients, alternating with the sour cream mixture and buttermilk, starting and ending with dry ingredients. Mix until just combined.

Step 2: Bake the Cake

1. Divide batter evenly among the prepared cake pans and smooth the tops.
2. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
3. Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Step 3: Make the Frosting

1. In a large mixing bowl, beat cream cheese and butter together until smooth.
2. Gradually add powdered sugar, mixing on low speed to prevent splattering.
3. Add key lime juice, zest, and vanilla extract. Beat until light and fluffy.

Step 4: Assemble the Cake

1. Place the first cake layer on a serving plate. Spread a generous amount of frosting on top.
2. Repeat with the second and third layers.
3. Apply a thin crumb coat of frosting over the entire cake and chill for 30 minutes.
4. Apply the final layer of frosting, smoothing it with an offset spatula.