Preheat oven to 350 degrees F and line two large baking sheets with parchment paper.
Combine the flour, salt, and baking soda in a bowl and whisk together. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter until light in colior, about 3 minutes.
Add the sugar and mix on high until light and fluffy.
Add the egg and vanilla, scraping down the sides of the bowl. Continue mixing until well combined.
Turn mixer to low and add the flour mixture and sour cream in three alternating batches starting and ending with the flour. Mix until just combined then use a spatula to scrape the bowl down.
Use a ¼ cup cookie scoop to drop balls of batter onto your lined baking sheets leave about 4 inches between each cookie as they spread in the oven. I recommend no more than 5 per baking sheet. You’ll want just under ¼ cup of batter for each cookie.
Bake on the center rack at 350 F for 13-14 minutes or until the centers are springy to the touch. Cool on pan for a few minutes before transferring to a wire rack to cool completely. Place the cookies upside down to cool so the bottom becomes the flat top you will ice.
When the cookies are cool combine the powdered sugar, vanilla, lemon juice, 1 tablespoon corn syrup and water in a large bowl then mix until smooth, adding more water as needed to a thinner spreadable icing.
Transfer half the icing to another bowl and add 3 tablespoons of cocoa powder and a few teaspoons of water and 2 teaspoons of corn syrup. Mix until glossy and smooth adding more water if needed to match the consistency of the vanilla icing.
Use a small spatula or knife cover half of each cookie in a thin layer of vanilla icing. Use the edge of a knife or offset spatula to create a neat line along the middle; just use the sharp edge to gently draw the icing from the center to the edge then clean the tool and repeat from the center to the other edge a second time. Allow the icing to set then cover the other side with the chocolate icing. This time use your spatula to push the chocolate right to the center then give it a quick smooth and allow to set completely.