Preheat your oven to 350°F (175°C). Grease and flour a cake pan.
In a mixing bowl, cream the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Mix in the key lime juice.
In another bowl, whisk together the flour, baking powder, baking soda, and salt.
Add the dry ingredients to the wet ingredients alternately with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared cake pan and bake for 30 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then turn it out onto a wire rack to cool completely.