Preheat your oven to 325°F and thoroughly grease a 9x5 inch loaf pan.
In a medium bowl, whisk together the almond flour, protein powder, baking powder, and salt until well combined.
In a large bowl, beat the softened butter and granular sweetener until the mixture is light and fluffy, about 2 minutes. Then, mix in the eggs one by one until fully incorporated.
Gradually add half of the dry almond flour mixture to the wet ingredients, mixing gently until just combined.
Next, beat in the key lime juice, water, and key lime zest.
Finally, mix in the remaining almond flour mixture until your batter is smooth and uniform.
Pour the batter into your prepared loaf pan and smooth the top evenly.
Bake for 55 to 65 minutes. To prevent the top from over-browning, gently cover the cake with foil about halfway through the baking time. The cake is perfectly done when the edges are golden and the top feels firm to the touch.
For the Glaze: In a small bowl, whisk the powdered sweetener and key lime juice together until smooth and pourable.
Once the cake has cooled completely, drizzle the vibrant glaze evenly over the top.