Preheat oven to 350 degrees F (176 C) and line a standard-size loaf pan with parchment paper.
To a large mixing bowl, add lemon zest, lemon juice, applesauce, avocado oil, maple syrup, and vanilla extract and whisk to combine.
Add almond flour, potato starch, cornstarch, baking powder, and sea salt and whisk again until well incorporated and no lumps remain. The texture should be thick and look similar to cornbread batter (see photo). If too thick, thin with a little bit of dairy-free milk. If too thin, add a little more almond flour.
Scoop the batter into the prepared loaf pan and spread it out evenly with a spatula. Transfer to the oven and bake for 35-40 minutes, or until the top is golden brown and an inserted toothpick comes out clean.
Let the cake cool completely in the pan — about 1 hour. Wash your mixing bowl to prepare glaze.
While the cake cools, make the glaze in the clean mixing bowl. Zest lemon and set aside. Whisk together vanilla extract and lemon juice, then begin adding sifted powdered sugar 1/4 cup (28 g) at a time, mixing until no lumps remain. Once all the powdered sugar is added, add zest and mix once more. Set aside.
Once the cake is completely cooled, remove from the pan and gently remove parchment paper. Place on a serving plate and, starting near one end of the cake, slowly pour glaze over the cake, letting it drizzle down the edges slightly.
Enjoy immediately with a cup of tea or some dairy-free milk! Store leftovers covered at room temperature for up to 2-3 days (we find it best within the first 1-2 days). Cake can be frozen for up to 1 month, ideally without glaze. Let thaw before slicing.
*We did not test other starches, but in place of potato starch, a blend of coconut flour and arrowroot starch may work. Or possibly oat flour. *In place of cornstarch, arrowroot may work. *Nutrition information is a rough estimate calculated with all of the glaze. *Prep time includes cooling.