Preheat your oven to 350°F (175°C) and coat two small loaf pans with non-stick baking spray.

In a large mixing bowl, combine the vanilla pound cake mix, cold water, melted unsalted butter, large egg, fresh key lime juice, key lime zest, and optional lime gel food coloring; stir until smooth.

Divide the batter equally between the two prepared loaf pans and gently tap them on the countertop.

Bake for 30 to 35 minutes, or until a toothpick inserted comes out clean.

Let the cakes cool in the pans for about 10 minutes before transferring to a wire rack to cool completely.

Prepare the glaze by whisking together powdered sugar, key lime juice, and key lime zest until smooth.

Once the cakes are cooled, pour the glaze evenly over the tops and allow it to set before slicing.