1. Preheat the oven to 325 F. Begin by running the sugar through a food processor a few times (pulsing it) to make it extra fine. You can also use a high-powered blender. Set aside.
2. Over a mixing bowl, sift the flour 4 times. You want it to be extra light and fluffy! To the bowl of a stand mixer fitted with the whisk attachment, or using a handheld electric mixer, add the salt, cream of tartar and egg whites. Whisk on high for a couple of minutes until the mixture thickens up and begins to form soft peaks. With the mixer running on low, sprinkle the sugar over the egg whites. Turn up the mixer to high and beat for an additional few minutes until the mixture is thick, shiny and forming stiff peaks. Add in the lemon zest, vanilla and lemon extract, and continue to beat until very thick, about 3-4 minutes.
3. Using a spatula, add in the flour 1/3 at a time, folding it gently in between additions to incorporate. Do not mix the batter too thoroughly. Once all the flour is added, pour the batter into an angel food tube pan and smooth out the top with the spatula. Give the pan a little shake to even things out. Bake the cake for 40-45 minutes. Check it at 40 minutes by lightly poking it with a finger. If it springs back, the cake is done. You can also use a toothpick!
4. Remove the cake from the oven, then immediately invert the pan, either on to a wire rack or on top of a glass. This will keep the cake’s height while it cools. Let the cake cool upside down for at least 1 hour, then run a knife gently around the outside edge of the pan before releasing the cake out on to a cake stand.
5. Prepare the glaze. In a glass measuring cup, add the powdered sugar, lemon juice and zest, and salt. Stir to combine. Add the butter, then heat the entire mixture in the microwave for 30 seconds. Give the glaze a stir, then pour it all over the cake. Serve immediately!

The cake (and glaze) will keep at room temperature for 2-3 days in an airtight container.