Preheat oven to 350F and line two baking sheets with silicone mats or parchment paper. Combine the flour, salt and baking soda in a bowl then whisk together and set aside.
Cream the butter in the bowl of your stand mixer fitted with a paddle attachment, then add the sugar and mix on high until light and fluffy. Mix in the egg and vanilla then scrape the bowl down and mix once more until well combined.
Add the flour mixture and buttermilk in three alternating batches starting and ending with the flour, while mixing on low. Mix until just combined then use a spatula to scrape the bowl down and mix in any errant bits of flour and butter.
Use a ¼ cup cookie scoop or greased measuring cup 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 350F for 13-14 minutes or until the centers are springy to the touch. Allow to cool for a few minutes before transferring to a wire rack to cool completely. Cool the cookies upside down so the bottom becomes the flat top you will ice.
While the cookies cool combine the powdered sugar, vanilla, lemon juice, corn syrup and water in a large bowl then mix until smooth, adding more water as needed to a thinner spreadable icing.
Transfer almost half the icing to another bowl and add 3 tablespoons of cocoa powder and a few teaspoons of water and a teaspoon of corn syrup. Mix on high until glossy and smooth adding more water if needed to match the consistency of the vanilla icing.
Use a small spatula or offset spatula to 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. 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 before enjoying.