• To make cake: Preheat oven to 350 degrees.
• Grease a cake pan and set aside.
• In a medium bowl, whisk the flour with the baking powder, baking soda and salt.
• Reduce speed to medium. Add half of the sugar and beat about 5 minutes. Increase speed to high and add the rest of the sugar. Beat for 5 seconds, then reduce speed to medium.
• In a small bowl, mix together the buttermilk, vanilla, and lime zest and juice  . Set aside.With an electric mixer, cream butter and granulated sugar. Add eggs one at a time.Add flour mixture, alternating with additions of the buttermilk mixture.
• Pour batter in a cake pan and bake until a toothpick inserted in center of a cake comes out clean, about 20 to 25 minutes.Let it cool on a rack.

Frosting

• Beat butter with a mixer on medium-high speed until creamy, about 1 minute.
• Reduce speed to medium. Add half of the sugar and beat about 5 minutes. Increase speed to high and add the rest of the sugar. Beat for 5 seconds, then reduce speed to medium.
• Add sweet condensed milk and beat until the frosting is smooth.
• Frost cooled lime cake and serve.