2. 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 reserved flour, and mix until just combined.
3. Reserve 1/2 cup batter; place in a pastry bag fitted with an #2 tip, and set aside for piping chocolate tuile cookies.
