Lemon and chocolate are a lovely and lively pair in this roulade. If you prefer milder lemon flavor, you can omit the rind from the filling. Or, if you are a bold lemon lover (like me), don't strain the rind out of the filling--enjoy the tartness and texture it provides.
Preheat oven to 350°. Lightly coat a jelly-roll pan with baking spray. Line bottom of pan with wax paper. Coat paper with baking spray. Set aside.
To prepare cake, weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour, cocoa, baking powder, and salt in a bowl; stir with a whisk. Place egg yolks, 1/4 cup granulated sugar, and vanilla in a large bowl; beat with a mixer at medium speed until light and fluffy (about 4 minutes). Place egg whites and cream of tartar in a medium bowl; beat at medium speed until foamy using clean, dry beaters. Beat mixture at high speed until soft peaks form. Gradually add 1/2 cup granulated sugar, 1 tablespoon at a time, beating until stiff peaks form (do not overbeat). Stir one-fourth of egg white mixture into egg yolk mixture; gently fold in remaining egg white mixture. Sift half of flour mixture over top of egg mixture; gently fold in. Sift remaining flour mixture over top of egg mixture; gently fold in. Spread mixture evenly into prepared pan.
Bake at 350° for 9 minutes or until cake springs back when lightly touched in the center. Loosen cake from sides of pan; turn out onto a dish towel dusted with 2 tablespoons powdered sugar. Carefully peel off wax paper; cool 1 minute. Starting at short side, roll up the cake and towel together. Place, seam side down, on a wire rack; cool completely (about 1 hour).
To prepare filling, place 1/3 cup granulated sugar, rind, juice, 2 tablespoons butter, and salt in a small saucepan over medium heat. Cook until butter melts and sugar dissolves, stirring frequently (about 4 minutes). Place remaining 1/3 cup granulated sugar, cornstarch, egg yolks, and eggs in a bowl; stir well with a whisk until smooth. Drizzle hot juice mixture into egg mixture, stirring constantly with a whisk. Return mixture to pan. Cook over medium heat until mixture thickens and reaches 180° on a candy thermometer (do not boil). Pour mixture through a sieve into a small bowl, pressing on solids. Discard solids. Add remaining 1 tablespoon butter, stirring until butter melts and is combined. Cover surface of mixture with plastic wrap. Chill completely.
To assemble roulade, gently stir whipped topping into chilled lemon filling. Unroll cake carefully; remove towel. Spread lemon filling over cake, leaving a 1-inch border around outside edges. Reroll cake; place, seam side down, on a platter. Cover and chill 1 hour. Sprinkle cake with 1 tablespoon powdered sugar. Cut cake into slices.
I opted to not roll this cake but instead used two 8" round cake pans and
put the filling in the middle more like a traditional cake. Baking
time was about 25 minutes. I really, really liked the lemon filling and
would use that again. But while the cake part was tasty, it very airy
and light. Which makes sense if you were going to try and roll it. I
wasn't sure about the lemon and chocolate combo, which is sort of odd,
but not bad. It is a lot of work and uses a lot of eggs. Not sure I'd
make this one again but I'm glad I tried it.