1. With a hand or stand mixer, cream the room temperature butter with the sugar until pale and fluffy, about 4 minutes.
2. Scrape the sides of the bowl and add the eggs and egg yolk one at a time, mixing well after each addition.  Stir in vanilla extract.
3. In a separate bowl, stir together flour, baking powder, baking soda, salt, and lemon zest.
4. Add the dry ingredients and the buttermilk in 3 additions, alternating each one and keeping the mixer on low speed.  Stop when just a few flour streaks remain.
5. Use a nonstick spatula to scrape the sides and bottom of the bowl, folding the mixture gently until remaining flour has been combined.
6. Chill dough for 1-3 hours.
7. Preheat oven to 350F and position rack in the center of oven.  Prepare 2 or 3 large baking sheets with parchment paper.  Do not grease.
8. For huge cookies: Use a 1/4 cup ice cream scoop and place level scoops of dough onto a prepared baking sheet, leaving ample space (at least 3 inches) between each one.  You'll probably only be able to get 4 or 6 cookies per sheet.  Bake for 8-10 minutes or until the cookies spring back when you gently press on the top.
9. For Regular Cookies: Use a medium cookie scoop (1.5 TBS capacity) to place level scoops of dough on baking sheet, leaving at least 2 inches between each one.  Bake for 8-10 minutes or until the cookies spring back when you gently press on the top.
10. Let the cookies cool for a few minutes on the sheet while you prepare the next batch with a cool sheet.  After a few minutes, transfer cookies to cooling rack to finish cooling and complete steps until all batter has been used.  Return dough to refrigerator between batches to keep the dough chilled.
11. Allow the cookies to completely cool before attempting to frost them.

Make the Icing:

1. Mix the powdered sugar, corn syrup, vanilla extract, and hot water together until fully combined.  The mixture should not be too thick, but should not be watery.  If it's too thin, add a little extra powdered sugar.  If it's too thick, add a little extra hot water.
2. Transfer half of the icing into a second bowl.  Add lemon juice to one of the bowls and stir - this will be the white frosting.
3.  Melt the chocolate in a microwave-safe bowl in 30-second increments, stirring after each one, until totally melted.  Cool for a minute, and then stir into the second bowl of icing.

Frost the Cookies:

1. On wax paper, flip the cookies over so that the flat side is facing up.  This is the side you will frost.
2. Use a small frosting spatula to scoop up a small amount of the white frosting and frost one side of each cookie.
3. Let the frosting set for at least 10 minutes and then frost the other side with the chocolate frosting.  You may need to give the frosting a good stir to keep it loose.
4. Let frosting set up fully and then store cookies in an airtight container with sheets of wax paper between layers.