Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan.
In a large mixing bowl, cream the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition.
Stir in the key lime juice and lime zest until well combined.
In another bowl, mix the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour the batter into the prepared cake pan and smooth the top.
Bake for 25-30 minutes or until a toothpick inserted comes out clean.
Let the cake cool in the pan for 10 minutes before transferring it to a wire rack.
While cooling, whip the heavy cream with powdered sugar and vanilla until soft peaks form.
Once the cake is cool, frost it with the whipped cream mixture and garnish with lime slices.