How to Make It
Preheat oven to 400°F. Spray a 10- x 15-inch rimmed baking sheet with cooking spray; line with parchment paper. Spray paper with cooking spray.
Prepare the Cake: Beat egg yolks and 1⁄2 cup of the granulated sugar with an electric mixer on high speed until thick and pale, 3 to 4 minutes. Beat in vanilla.
Using clean, dry beaters, beat egg whites and cream of tartar in a separate bowl on medium speed until foamy. Increase speed to high, and beat until soft peaks form. Add remaining 1⁄4 cup sugar, 1 tablespoon at a time, beating on high speed until medium peaks form. Stir one-fourth of egg white mixture into egg yolk mixture; gently fold in remaining three-fourths egg white mixture.
Sift together cake flour, baking powder, and salt. Sift one-third of flour mixture over egg mixture, and sprinkle with 1 tablespoon of the sprinkles. Fold gently until just blended. Repeat twice with 2 tablespoons of the sprinkles and remaining flour mixture. Spread batter gently in prepared pan, taking care not to deflate batter
Bake in preheated oven until puffed and lightly browned on top, 7 to 9 minutes. Run a butter knife or offset spatula around edges of pan to loosen; cool in pan 2 minutes. Sprinkle 4 tablespoons of the powdered sugar over top. Invert onto a parchment paper-lined surface. Gently peel top layer of parchment paper from Cake; sprinkle top with remaining 2 tablespoons powdered sugar. Starting at 1 short side, roll Cake and bottom parchment together. Transfer to a wire rack, and cool completely, about 1 hour.
Prepare the Filling: Place water in a small microwavable bowl, and sprinkle with gelatin; let stand 5 minutes. Microwave on HIGH until gelatin melts and mixture is smooth, about 10 seconds. Let stand 5 minutes.
Place cream, powdered sugar, and, if desired, liqueur in a large bowl. Beat on medium-high speed until soft peaks form. With mixer on low speed, slowly drizzle in gelatin. Increase speed to high, and beat until stiff peaks form. Cover and chill until ready to use.
Unroll Cake onto a flat surface. Spread Filling over top, leaving a 1⁄2-inch border on all sides. Starting at 1 short side and using parchment paper as a guide, roll up Cake in a jelly-roll fashion, discarding parchment. Place on a platter, seam side down.
Prepare the Frosting: Beat butter with an electric mixer on medium speed until creamy and smooth. Add powdered sugar, lemon juice, and salt; beat until smooth. Beat in cream, 1 tablespoon at a time, until mixture is a spreadable consistency. Spread Frosting over top and sides of roulade, leaving ends uncovered. Sprinkle remaining 1 tablespoon candy sprinkles over top and sides of roulade.