3cupschocolate chips(18 ounces) (milk chocolate is recommended; I include at least half milk chocolate chips, or else the frosting is too bitter)
1 ¼cupheavy cream(283 grams)
¼teaspoonsalt
1 ⅓ to 1 ⅔cupscocoa powder(142 grams, which for me was 1 ⅓ cups)
1 ¼cupspowdered sugar(142 grams, which for me was more like 1 cup)
1 ½teaspoonvanilla(7 grams)
Instructions
Sugar Cookie
With an electric mixer, cream butter and sugar for 3 minutes, until fluffy. Scrape down the sides, then beat in egg white, 1 ½ tablespoon heavy cream, and 1 ½ teaspoon vanilla. Turn the mixer down to low; add the flour, cornstarch, baking powder, and salt in 3 additions, mixing until just combined.
Scoop into 1-tablespoon balls (a 2 teaspoon (#60) cookie scoop works well here) and roll with your hands. Place them 2 inches apart on a cookie sheet. Flatten each ball into a 2-inch round about ¼ inch tall. Bake at 350F for 8-10 minutes, until bottoms are just starting to brown. Cool completely on the baking sheet.
Chocolate Ganache Frosting
Combine the chocolate chips, cream, and salt in a large microwave-safe bowl. Microwave in 15 second bursts, stirring after each one, until chocolate is melted and smooth. Use a whisk to stir in cocoa powder, powdered sugar, and vanilla, mixing until smooth and shiny. The frosting will be thick, like brownie batter.
(Frosting should be between 90-100F. It will form mounds on the cookies and may spread a little, but shouldn't run off. Heat frosting for 5 seconds in the microwave if it's too thick to spread. If it spreads too much, let it cool.)
Turn cookies over on the baking sheets. Spoon 2 tablespoons of frosting onto the flat side of each cookie. (I used a 1 ½ tablespoon (#40) scoop and it worked perfectly.)
Let set at room temperature for 1-3 hours. We happily ate them after 1 hour because we couldn't stand to wait, but I wouldn't stack or store them until 3 hours. Store in a container at room temperature for 3 days (if you don't eat them all immediately, which will be tough).
Notes
Using mostly or all semisweet chocolate in these cookies makes the chocolate part rather dark. I found the frosting a little bitter when I only used 4 ounces milk chocolate. I recommend using at least half milk chocolate chips, if not more.Adapted from Brown Eyed Baker