In your mixing bowl, combine the granulated sugar and the 2 tablespoons of Key Lime zest. Use your fingertips to rub the zest into the sugar for 2 minutes until the sugar is fragrant and damp. This releases the oils that baking alone cannot extract.
Add the softened butter and 8 oz of cream cheese to the lime sugar. Beat on medium-high speed for at least 5 minutes until the mixture is pale and incredibly fluffy.
Add the eggs one at a time, beating well after each addition. Scrape the sides of the bowl to ensure the fat and proteins are fully emulsified. Stir in the vanilla bean paste.
Whisk the flour, salt, and baking soda. Reduce the mixer speed to low. Add the flour mixture in three parts, alternating with the Key Lime juice, beginning and ending with the flour. Do not overmix; stop as soon as the last streak of flour disappears.
Pour the batter into a heavily greased and floured Bundt pan. Bake at 325°F for 60–70 minutes. The lower temperature is vital for a pound cake of this density to rise evenly without a thick, dry crust.
Let the cake cool in the pan for exactly 10 minutes before inverting it onto a wire rack. Cooling it completely before icing is mandatory to prevent the cream cheese from melting.
Whisk the icing ingredients until smooth. Pour over the cooled cake, allowing it to drip down the sides. Top with fresh zest.