Preheat oven to 375º. Line 10”x15” jellyroll pan with parchment paper. Whip all filling/frosting ingredients together in large bowl until thick and stiff. Refrigerate.
Use electric mixer to beat egg yolks with ½ cup sugar until frothy and light. Blend in ⅓ cup cocoa, 1 ½ tsp. vanilla, salt.
Using clean beaters and separate bowl, whip egg whites to soft peak stage. Gradually add ¼ cup sugar, beat into stiff peaks.
Immediately fold yolk mixture into egg whites, then spread batter evenly into prepared pan.
Bake until cake springs back when lightly touched, 10-12 min.
Dust clean dishtowel with powdered sugar. Run knife around edge of pan, turn warm cake out onto towel. Remove parchment paper.
Starting at short edge of cake, roll up with towel. Cool 30 min.
Unroll cake, spread filling to within 1” of edges.
Roll cake up with filling inside. Place seam-side down onto serving plate, finish as desired with powdered sugar, leftover filling, festive accents. Refrigerate until serving.
