Recipe from Episode 703 of America's Test Kitchen's Cook's Country. Video of that segment can be viewed here. Note my tips and modifications.

Wikipedia and the NYT offer a more detailed history of these cookies - not to mention the 10 Things you didn't know about them. Many of us first heard about B&W Cookies on Seinfeld's "Look to the cookie!" episode. They have been described as "cake-y" cookies or in Wiki "a soft, sponge-cake-like shortbread". For sure they are not hard crunchy cookies. In NYC, first appearances date back to the turn of the last century, and are credited to Hemstrought Bakery (now closed) and Glaser's Bake Shop. It's not hard to find NYC articles aimed at discovering / debating the best B&W cookie - Food52 and NYEater and SeriousEats. Photos reveal that some have a pretty dull frosting, while others are glazed (as in this recipe). Some say the original cookies had a fondant icing. There are many versions of the recipe - and even ATK offered up an older one, different than the one I used. Smitten Kitchen has offered her version. Even Martha Stewart has more than one version, and this is the one I used to use. Nothing wrong with Martha's recipe but I now tend to use this one. Please read my Tips before beginning.

Makes 12 really large cookies - or more if smaller... see my Tips below. Remove egg and butter from refrigerator and let butter soften; make sure you have sour cream. Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 F. Line 2 cookie sheets with parchment paper. 

1 3/4 cups (8 3/4 ounces) all-purpose flour  
1/2  tsp baking powder  
1/4  tsp baking soda  
1/8  tsp salt 

In a bowl, stir together flour, baking powder, baking soda, and salt. 

10 TB unsalted butter, softened  
1  cup (7 ounces) granulated sugar  
1  large egg  
2  tsp vanilla extract  
1/3 cup sour cream  

Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 2 minutes. Add egg and vanilla and beat until combined.

Reduce speed to low and add flour mixture in 3 additions, alternating with 2 additions of sour cream, scraping down bowl as needed. Give dough final stir by hand to ensure there are no streaks of flour. 

ATK suggests using a greased 1/4 cup measure, dropping cookie dough 3 inches apart onto prepared baking sheets. I use a scoop sized 30 - 40 ml (see Notes below).

Bake until edges are lightly browned, 15 to 18 minutes, switching and rotating sheets halfway through baking. Let cookies cool on sheets for 5 minutes, then transfer to wire rack to cool completely before glazing, about 1 hour. 

5 cups (20 ounces) confectioners' sugar, sifted  
6 + 1 TB whole milk  (or milk / lemon juice combo - see Notes)
2 TB corn syrup  
1  tsp vanilla extract  
1/2  tsp salt  
3 TB Dutch-processed cocoa powder, sifted  

GLAZE. No point in making this ahead. Can be made while cookies are cooling (for 1 hour). If the glaze sets a bit before use, add more milk or lemon juice but only a few drops at a time.

Whisk sugar, 6 TB milk, corn syrup, vanilla, and salt together in bowl until smooth. Transfer 1 cup glaze to small bowl; reserve. Whisk cocoa and remaining 1 TB milk into remaining glaze until combined. (See Note below re lemon.)

Working with 1 cookie at a time, spread 1 tablespoon (or more) vanilla glaze over half of underside (flat side) of cookie. Here's my demo of how to glaze the cookies. Place each cookie on a rack over some parchment to minimize mess from drips. Refrigerate until glaze is set, about 15 minutes. Cover other half of cookies with 1 tablespoon chocolate glaze and let cookies sit at room temperature until glaze is firm, at least 1 hour - maybe longer.  

Notes and Tips...

  • 12 Cookies? -  B&W Cookies are often made/sold large - 4 - 6 inches. I have purchased mini cookies at NYC Whole Foods - have never seen them in WF here. The Martha Stewart recipe I used to use also made mini cookies - about 2" across. I prefer smaller cookies than large - even if I end up eating two small cookies - haha! - in fact, eating two is part of the beauty of small cookies. The ATK recipe is designed to make only a dozen cookies, with 6 on each cookie sheet. I use a smaller scoop - about a 1/8 cup measure - and get about 18-24. Pay attention to bake time if making smaller cookies.
  • Butter - I have mentioned before that ATK recipes use European Style butter (84% fat). I suspect that because this is a cake-y cookie, it would work fine with regular butter.
  • Cocoa - see details about Dutch Processed. Be sure to sift to remove lumps.
  • Bake time - 15-18 minutes was advised for the larger cookies. Since mine are slightly smaller I do 8, and then rotate pans, and then another 7. Remove when edges are lightly browned.
  • Glaze - B&W cookie aficionados claim that there must be a slight lemon flavour and I'd agree. Since this recipe did not call for lemon in the batter or the glaze, I substituted 2 TB of fresh lemon juice as part of the 6 TB milk - in fact I sometimes find I need to use a bit more liquid to get the ideal glaze consistency - not too runny, not too thick. . 
  • Applying glaze - For best results, the cookies do need to cool. The recipe yields a large batch of glaze so there's no need to be too skimpy. Obviously too thick is not ideal. IMPORTANT -  note that the glaze is applied to the flat/underside of the cookie!!
  • Store - The glaze "seems to harden" fairly quickly, but don't be fooled - it is still soft-ish and can be easily messed up. If not using the same day, I'd leave them out in a single layer overnight. Next day the glaze is shiny and firm enough to enable storage in layers separated by parchment.  (Cookies can be stored at room temperature for up to 2 days - if they last that long.)
  • Make ahead? I suppose one could make the cookies one day and then glaze them the next day.

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