Put a creative twist on the traditional rolled and filled cake known as roulade by going marble -- all you have to do to create the pretty pattern is mix up two contrasting batters.
Bring cream just to a boil in a small saucepan over medium heat; pour over chocolate in a medium bowl. Let stand until softened, about 3 minutes; using a rubber spatula, stir until emulsified. Cover with plastic and refrigerate overnight.
Butter a rimmed baking sheet; line with parchment, then butter parchment. Preheat oven to 350 degrees. In a medium bowl, whisk together flour, 1/4 cup sugar, baking powder, oil, egg yolks, and milk. In the bowl of a mixer fitted with the whisk attachment, combine egg whites, salt, cream of tartar, and vanilla. Whisk on medium speed until foamy, about 1 minute. Slowly add remaining 1/2 cup sugar. Increase mixer speed to high and continue whisking until stiff, glossy peaks form, about 5 minutes more.
Stir about one-third of egg-white mixture into flour mixture to lighten, then gently fold in remaining egg-white mixture in three additions. Transfer about 1 cup batter to a small bowl and gently fold in cocoa. Using an offset spatula, spread vanilla cake batter in prepared pan, filling pan completely and being careful not to deflate batter. Drop teaspoons of chocolate batter evenly across surface of vanilla batter, then swirl together to create a marble pattern. Bake until lightly golden brown and center of cake springs back when lightly pressed, about 18 minutes.
Run a knife between sides of cake and pan to loosen. Invert cake onto a sheet of parchment lightly coated with cooking spray; remove uncoated parchment from cake. Using coated parchment, start at a short end and roll up cake. Let cool completely, seam-side down.
Unroll cake. Whisk chilled chocolate filling to soft peaks. Evenly spread on top of cake, leaving a 1/2-inch border. Starting at a short end, carefully roll up cake without parchment. Transfer, seam-side down, to a serving platter to maintain cylindrical shape. Slice and serve (uncut rolled cake can be refrigerated up to 1 hour).