Preheat oven to 350°F
Line two baking sheets with parchment paper or silicone baking mats
In a medium bowl, whisk together flour, baking powder, and salt; set aside
Using an electric mixer, beat the butter and sugar on medium-high speed until pale and fluffy, about 3 minutes
Add the egg white, heavy cream, and vanilla, and beat until combined
Reduce speed to low and add flour mixture in three additions, mixing until incorporated, scraping down the bowl as needed
Working with 1 heaping tablespoon of dough at a time, roll into balls and space 2 inches a part on the prepared baking sheets
Using your hands, press down the tops of the balls slightly
Bake until the cookies are just beginning to brown around the edges, 8 to 10 minutes, rotating the sheet halfway through baking
Let the cookies cool completely on the baking sheet
Once the cookies have cooled, combine the chocolate chips, heavy cream, and salt in a large bowl
Microwave the chocolate mixture at 50 percent power, stirring occasionally, until melted and smooth, 1 to 3 minutes
Whisk the cocoa powder, powdered sugar, and vanilla into the chocolate mixture until smooth, the frosting should the texture of thick brownie batter and register about 95 degrees.
Turn the cookies over on the baking sheets
Spoon 2 tablespoons of frosting over the flat side of each cookie to form a mound
Let the cookies sit at room temperature until the frosting is set, about 3 hours
The cookies can be stored in an airtight container at room temperature for up to 3 days