New York City Black and White Cookies

Makes 12-15 large cookies

2 c all-purpose flour
1 tsp baking powder
1/2 tsp salt
1/4 tsp baking soda
1/3 c full-fat sour cream use 10% fat yogurt in a pinch
1/3 c buttermilk or whole milk soured by replacing 1 tbsp of the milk with vinegar or lemon juice
1 tsp vanilla
1/2 c unsalted butter room temperature
2 tsp finely grated lemon zest
3/4 c sugar
2 large eggs room temperature

Preheat oven to 350 and line two cookie sheets with parchment.
In a medium bowl, whisk the four dry ingredients [flour, powder, salt and soda] and set aside.
In a separate bowl, mix the sour cream, buttermilk, and vanilla, and set aside.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream the butter, lemon zest, and sugar until light and fluffy.
Add the eggs, one at a time, and beat until combined.
Turn the mixer to low and alternately beat in about 1/4 of the flour mixture and 1/4 of the sour cream mixture, repeating until combined, and no pockets of flour remain. [The cookie dough will be very thick.]
Using a 1/4 cup measure or scoop, dollop the batter 3 inches apart on prepared cookie sheets. With wet fingers or the wet bottom of the measuring cup, flatten and spread the dough into a 3-inch diameter circle. You should have about 6-7 cookies per sheet.
Bake for 16-18 minutes, rotating the sheets after 8 minutes of the bake, until tops are puffy and just golden. [Bottoms may be slightly more golden brown.]
Allow cookies to cool on the pan for 10 minutes before moving to wire racks to cool completely.

In a medium bowl, whisk the icing sugar with 3 tbs of the boiling water, corn syrup, and vanilla.
Add boiling water by the tablespoon, stirring constantly, until you have a thick, spreadable icing, like a thick caramel sauce. [You can always thin it out with a bit more hot water as you go.]
Ice half of the flat golden BOTTOM of each cookie with the vanilla icing, allowing to cool on the counter, bottoms up, for 30 minutes or refrigerating for 15, while you make the chocolate batch. [You will likely use less than half of the vanilla mixture!]
Into the remaining vanilla icing mixture, whisk the cooled melted chocolate.
Then, whisk the cocoa into the chocolate icing, adding more boiling water by the tablespoon, as necessary, to achieve a thick, spreadable icing with the same consistency as the vanilla, ie a nice caramel sauce.
Frost the bare halves of the bottoms of each cookie with the chocolate icing.
Allow the icing to harden for an hour before serving.
Cookies will stay fresh in an air-tight container for two days or sealed in the refrigerator for about a week. You can also place the cooled and iced cookies into a freezer bag and freeze for two to three months, thawing overnight in the refrigerator and then on the counter.