Preheat your oven to 350 degrees. Get two cookie sheets lined with a Silpat and set them aside.
Take your stand mixer and add the vegan butter, oil, sugars, vegan milk, and egg replacer. Mix this on medium until it's creamed together. This will take about 2 minutes.
Now add your baking soda, cream of tartar, and salt. Mix this in until incorporated. Now add your 5 ½ cups of flour and mix until combined. This is a dryer dough. It should not be sticky at all.
Get a small bowl and mix the ¼ cup sugar and dash of salt and stir, set aside.
Take your small cookie scoop and scoop balls with the dough and place them on the cookie sheet; mine fits 12 cookies. Now take the bottom of a round glass, dip the base into the sugar/salt mixture, then gently smash the cookie down; you don't want to smash it flat, just enough to make it flat (look at the pictures for reference). Repeat with the remaining cookies on the cookie sheet. Cook for 7- 8 minutes, don't overcook these. They will firm up some on the counter. Let rest on the cookie sheet for a few minutes, and then place it on the cooling rack until cool. Repeat until the cookies are gone.
While the cookies are cooling, make your frosting. Take your cleaned-out stand mixer and mix your room-temperature vegan butter, vegan sour cream, powdered sugar, salt, and vegan milk until fluffy, for about 2 minutes. This makes just enough to frost all the cookies. I don't like a huge amount of frosting, so if you really want to stack it on the cookies, I would 1.5 times the frosting, but this makes just enough for me.