For the Key Lime Cake:

1. Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line them with parchment paper.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
3. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-4 minutes.
4. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla extract, key lime juice, and key lime zest.
5. Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Start and finish with the dry ingredients. Mix until just combined.
6. Divide the batter evenly between the two prepared cake pans and smooth the tops with a spatula.
7. Bake for 25-30 minutes, or until a toothpick inserted into the center of the cakes comes out clean.
8. Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

For the Key Lime Cream Cheese Frosting:

1. In a large bowl, beat together the softened cream cheese and butter until smooth and creamy.
2. Gradually add the powdered sugar, one cup at a time, and continue to beat until the frosting is fluffy.
3. Mix in the key lime juice, key lime zest, vanilla extract, and a pinch of salt until well combined.
4. If the frosting is too thick, you can add a tablespoon of milk to reach your desired consistency.

Assembling the Cake:

1. Once the cakes have cooled, spread a layer of frosting on top of the first cake layer.
2. Place the second cake layer on top and frost the entire cake with the key lime cream cheese frosting.
3. Garnish with additional lime zest or thin lime slices for decoration, if desired.