The consistency of the frosting should resemble that of a thick brownie batter. It should mound and slowly spread over the cookies. It’s OK if some of the frosting drips down the sides of the cookies. If the frosting’s temperature drops below 90 degrees, it may become too thick to spread. To bring it back to its proper consistency, simply microwave it at 50 percent power in 5-second intervals, whisking after each interval. Our favorite Dutch-processed cocoa powder is Droste Cocoa.
1. FOR THE COOKIES: Adjust oven rack to middle position and heat oven to 350 degrees. Line 2 baking sheets with parchment paper. Whisk flour, baking powder, and salt together in bowl; set aside. Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 3 minutes.
2. Add egg white, cream, and vanilla and beat until combined. Reduce speed to low and add flour mixture in 3 additions until incorporated, scraping down bowl as needed.
3. Working with 1 heaping tablespoon dough at a time, roll into balls and space 2 inches apart on prepared sheets, 12 per sheet. Using your moistened fingers, press dough balls to form disks about 1/4 inch thick and 2 inches in diameter. Bake, 1 sheet at a time, until cookies are just beginning to brown around edges, 8 to 10 minutes, rotating sheet halfway through baking. Let cookies cool completely on sheet.
4. FOR THE FROSTING: Once cookies have cooled, combine chocolate chips, cream, and salt in large bowl. Microwave chocolate mixture at 50 percent power, stirring occasionally, until melted and smooth, 1 to 3 minutes. Whisk cocoa, sugar, and vanilla into chocolate mixture until smooth. (Frosting should be texture of thick brownie batter and register about 95 degrees.)
5. Flip cookies on sheets. Spoon 2 tablespoons frosting over flat side of each cookie to form mound. Let cookies sit at room temperature until frosting is set, about 3 hours. Serve. (Cookies can be stored in airtight container at room temperature for up to 2 days.)
