Preheat oven to 325°F (165°C). Grease a 10x5-inch loaf pan and line with parchment paper.
In a large bowl, cream softened butter and cream cheese with an electric mixer for 2–3 minutes.
Add sugar substitute and beat for another 2–3 minutes until light and fluffy.
Add eggs one at a time, beating well after each addition. Scrape down the sides of the bowl regularly.
Mix in vanilla extract until well combined.
In a separate bowl, whisk together coconut flour, baking powder, and sea salt.
Gradually add dry ingredients to the wet mixture, mixing on low just until combined.
Gently fold in sour cream by hand using a spatula. Batter will be thick.
Transfer batter to the prepared loaf pan and spread evenly.
Bake for 75–90 minutes, or until a toothpick inserted into the center comes out clean with just a few crumbs.
Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.
If using lemon glaze, whisk all glaze ingredients together and pour over the cooled cake. Let set before slicing.