Preheat oven to 325°F (163°C). Grease and flour a 10-inch bundt pan or two 9x5 loaf pans and set aside.

In a large bowl, beat 1 cup softened butter with 2 cups sugar until light and fluffy, about 3–5 minutes. Scrape the bowl.

Add eggs one at a time, beating briefly after each addition so the batter stays emulsified. Stir in 1 teaspoon vanilla.

Whisk together 3 cups flour, 1/2 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon salt.

In a separate bowl, combine 1 cup sour cream, 1/4 cup key lime juice, and 1 tablespoon lime zest.

Add the dry ingredients to the butter mixture in three additions, alternating with the sour cream mixture (dry, wet, dry, wet, dry). Mix each addition just until incorporated.

Scrape the batter into the prepared pan(s), smooth the top, and tap gently to remove large air pockets.

Bake at 325°F for about 60–75 minutes for a bundt, or 55–65 minutes for loaves. Start checking at 55 minutes.

Cool in the pan for 10–15 minutes, then invert onto a rack to cool completely before icing.

Beat 8 oz softened cream cheese and 1/4 cup softened butter together until smooth.

Gradually add 4 cups powdered sugar, adding 2 tablespoons key lime juice and 1 teaspoon lime zest. Adjust consistency as needed.

Once the cake is completely cool, spread or drizzle the icing over the top. Chill briefly to set the icing, then slice and serve.