1 c All-purpose flour
3 tb Dutch-process cocoa powder
1/2 c Unsalted butter; room temperature
2/3 c Confectioner's sugar
4 lg Egg whites; room temperature
Sift flour with cocoa, and set aside.
In bowl of an electric standing mixer with paddle attachment, beat
butter and sugar on medium, until well combined, about 2 minutes.
Beat in egg whites, one at a time, beating for 4 minutes after each
addition. Add the flour-and-cocoa mixture, and mix until just
combined.
Reserve 1/2 cup batter; place in a pastry bag fitted with an #2
tip, and set aside for piping white tuile cookies.