Using a hand whisk or a stand mixer fitted with the paddle (low speed), cream butter, sugar and vanilla to a paste. Keep stirring while you gradually add the egg whites. Continue to add the flour in small batches and stir to achieve a homogeneous and smooth batter/paste. Be careful to not over-mix.
Mix a small amount of the batter with red food coloring and some with cocoa powder.
Cover the bowls with plastic wrap and chill in the fridge for at least 30 minutes to firm up. (This batter will keep in the fridge for up to a week, take it out 30 minutes before you plan to use it).
Line a baking sheet with a silicon mat. Place a stencil with a circle cut out (about 4" in diameter) on the mat. Press the stencil on the baking sheet and use an offset spatula to spread the chocolate and vanilla batters. Leave some room in between your shapes. Place the red batter in a paper piping bag and pipe lines on the circles.
Bake in a preheated oven (180C/350F) for about 5-10 minutes or until the edges turn golden brown. Immediately release from baking sheet and proceed to shape into cornettes. Keep them in an airtight container until ready to use.