How to Make It
Prepare the Snowflakes: Coat a rimmed baking sheet with baking spray, and line with parchment paper. Combine flour, sugar, butter, and egg white in a bowl; beat with an electric mixer on low speed until smooth. Transfer mixture to a disposable piping bag; snip a very small opening in the tip of the bag. Pipe snowflake designs onto prepared baking sheet.
Prepare the Cake: Preheat oven to 375°F. Sift flour and cocoa into a bowl. Beat egg yolks with an electric mixer fitted with whisk attachment on medium speed until pale yellow and thick, 2 to 3 minutes, adding 1/4 cup of the sugar in 3 additions. Continue to beat on medium speed until thick and fluffy, 2 to 3 minutes. Beat in vanilla and salt. Using a clean, dry whisk attachment, beat egg whites in a bowl on medium speed until foamy, 1 to 2 minutes; increase speed to high, and beat until stiff peaks form, 1 to 2 minutes. Add remaining 1/4 cup sugar, 1 tablespoon at a time, beating well after each addition. Beat on high speed until stiff peaks form, about 5 minutes. Gently fold one-third of egg white mixture into yolk mixture. Gently fold in remaining egg white mixture. Sprinkle flour mixture over top of egg mixture; gently fold in flour mixture being careful not to deflate batter. Gently spread batter on top of snowflakes in prepared pan; quickly smooth batter with a spatula. Bake in preheated oven until puffed and firm, 8 to 9 minutes.
Run a knife around edges of pan to loosen cake; turn cake out onto a large lint-free kitchen towel. Place a second towel over top of cake. Invert cake and towels so the parchment side of the cake is facing you; remove first towel. Starting at 1 short side, roll up cake, parchment, and towel together, jelly-roll fashion. (Snowflake pattern will be on the outside.) Place rolled cake on a wire rack until completely cool, about 30 minutes.
Prepare the Filling: Place powdered sugar, butter, cream, salt, and peppermint extract in a large bowl; beat with an electric mixer on low speed until combined. Increase speed to medium, and beat until smooth and creamy, 1 to 2 minutes. Add food coloring, 1 drop at a time, until desired color is reached.
Assemble the Roulade: Carefully unroll cake on a work surface. Carefully remove parchment, and invert cake again as before. Spread Filling over surface of cake (on the side without snowflakes) to within 1/2-inch of edges. Sprinkle crushed peppermint candies over Filling. Re-roll cake, starting at short side. Place cake roll on a serving plate, seam side down. Trim ends of cake if desired. Using a very fine mesh sifter, sprinkle with powdered sugar.