Preheat your oven to 350°F (175°C) and grease two 9-inch round cake pans.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
Add softened butter and beat until the mixture resembles coarse crumbs.
Add eggs, buttermilk, key lime juice, and zest; mix until smooth.
Divide the batter evenly between the prepared pans.
Bake for 25-30 minutes or until a toothpick inserted comes out clean.
Allow cakes to cool in pans for 10 minutes before transferring to wire racks.
For frosting, whip heavy cream until soft peaks form; gradually add powdered sugar and vanilla.
Once cooled, spread frosting between layers and on top of the cake.
Decorate with lime slices and zest as desired.