Preheat the Oven: Set your oven to 300°F. While it heats, grease and flour a 10-inch tube pan and line the bottom with parchment paper.
Cream the Ingredients: In a large mixing bowl, combine white sugar, softened unsalted butter, and shortening. Beat until creamy and light, about 3-5 minutes.
Mix the Dry Ingredients: In a separate bowl, sift together the cake flour, baking powder, and salt. Gradually add this to the creamed ingredients along with the eggs and heavy cream, mixing on low speed until just combined.
Add Flavors: Stir in the key lime juice, lime zest, and vanilla extract.
Pour the Batter: Gently pour the batter into the prepared pan and smooth the top.
Bake the Cake: Bake for 1 hour and 30 minutes or until a toothpick comes out clean.
Cool the Cake: Remove from the oven and let cool in the pan for 15 minutes.
Prepare the Glaze: Whisk together powdered sugar, key lime juice, and vanilla extract until smooth.
Glaze the Cake: Once the cake is cool, brush the glaze over the top and sides.