3 tablespoons cocoa powder, preferably Dutch process
1/2 vanilla bean, split lengthwise
8 tablespoons (1 stick) unsalted butter
2/3 cup confectioners' sugar
4 large egg whites, room temperature
Make a 7 by 1 1/4-inch rectangular template by cutting a section out of a 1/8-inch-thick piece of cardboard.
Heat oven to 400 degrees F.
Sift 1/3 cup plus 1 tablespoon flour with the cocoa, and set cocoa-flour mixture aside. Using the edge of a paring knife, scrape seeds from the vanilla bean into the bowl of an electric mixer. Add butter and sugar. Using the paddle attachment, beat seeds, butter, and sugar on medium speed until fluffy, about 5 minutes. Beat in egg whites, 1 at a time.
Transfer half of the batter (2/3 cup) to a medium bowl. Fold in the cocoa-flour mixture until well combined; set aside.
Fold the remaining flour into the remaining batter until well combined.
Fit a baking sheet with a silpat nonstick baking mat or parchment paper and place the stencil on the mat. Using a small offset spatula, spread 1 tablespoon of white batter over the stencil. Lift stencil, and remove. Fill a pastry bag fitted with a #2 Ateco plain tip with 1/4 cup chocolate batter, and pipe your message onto the tuile. Repeat the process to make 25 white tuiles, reserving 1/4 cup white batter.
Bake tuiles 6 minutes. Using a spatula, remove from baking sheet, and lay over a rolling pin to curl. If tuiles become too cool before shaping, return to oven 30 seconds. Let cool completely on pin.
Repeat with chocolate batter, using reserved white batter for piping. Store tuiles in an airtight container for up to 1 week.