Preheat the oven to 375 degrees F.
Place the butter, sugar, salt, and vanilla into the bowl of a stand mixer with the paddle attachment. Mix at low speed. When combined, add egg white then barely incorporate the flour.
Divide the mixture in half. Add the cocoa powder to one half and the cornstarch to the other half. Place each batter a separate pastry bag, then place both bags inside a larger pastry bag fitted with a 3/4-inch star tip.
Pipe rosettes or drops onto a parchment paper lined baking sheet. Bake for 12 to 15 minutes.
Recipe courtesy of Jacques Torres