Preheat oven to 350℉ with rack in the middle position. Line two baking pans with silicon baking mats, or spray the pans with cooking spray, line with parchment, and lightly spray the parchment with cooking spray.
In a medium bowl, whisk together the flour, cornstarch and salt.
Using stand mixer fitted with paddle, beat butter and sugar on medium-high speed until pale and fluffy, about 2 minutes. With mixer running, slowly add egg whites and vanilla and mix until well incorporated, scraping down bowl as needed. Reduce speed to low, add flour mixture, and mix until just combine.
Drop 2-teaspoon portions of batter onto prepared sheets, spaced at least 6 inches apart. Using small offset spatula, spread each into 5-inch circle of even thickness. Bake, 1 sheet at a time, until edges are golden brown, 5 to 7 minutes.
Working quickly, lift edge of each cookie with offset spatula and roll tightly into cigar shape; place seam side down on wire rack. If cookies become too stiff to roll, return to oven for 30 seconds to soften. Repeat with remaining batter. Let cookies cool completely.
Microwave chocolate, stirring every 20 seconds, until melted. Dip about one-quarter of each cookie into chocolate, wiping away excess. Sprinkle evenly with nuts, if using; transfer to parchment-lined baking sheet; and let chocolate set, about 30 minutes, before serving.

Tuile Cookies

Swiss Meringue Buttercream