Preheat oven to 325°F (160°C). Grease and flour a 9x5 inch loaf pan with parchment paper, leaving an overhang on the long sides to easily lift the cake.

In a medium bowl, whisk together almond flour, protein powder, baking powder, and salt. Set aside.

In a large bowl, cream together softened butter and granular sweetener until light and fluffy. This should take about 3-5 minutes with an electric mixer.

Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next. Scrape down the sides of the bowl as needed.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Stir in the key lime juice, water, and key lime zest until evenly distributed. The batter will be thick.

Pour the batter into the prepared loaf pan and smooth the top. Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean. If the top browns too quickly, tent loosely with foil.

Let the cake cool in the pan for 15 minutes before carefully lifting it out using the parchment paper overhang. Transfer to a wire rack to cool completely before slicing and serving. For best flavor, chill for at least an hour.