How to Make It
Preheat the oven to 325°. Lightly butter two 10 1/2-by-15 1/2-inch jelly roll pans and line the bottoms with parchment paper. Butter the paper. Dust the pans with flour, tapping out any excess.
In a large bowl, using an electric mixer, beat the egg whites with 1/2 teaspoon of the cream of tartar at high speed until soft peaks form. Add 1/2 cup of the confectioners' sugar and beat at high speed until stiff and glossy.
In another large bowl, beat the egg yolks until pale, about 2 minutes. Add the flour, 3 tablespoons of cocoa, 1/2 cup of the confectioners' sugar and 1 teaspoon of the vanilla and beat at low speed until combined. Beat in one fourth of the egg whites, then fold in the remaining whites until no streaks remain. Pour the batter into the prepared pans and spread in a thin, even layer. Bake the cakes for 6 minutes, until firm and slightly puffed. Transfer the pans to a rack and let cool for 5 minutes.
Run the tip of a knife around the edge of the cakes. Dust the cakes with cocoa and cover each one with a sheet of wax paper slightly larger than the pan. Invert the cakes onto the wax paper and remove the pans. Cover the cakes with plastic wrap to keep moist.
In a small bowl, combine 2 tablespoons of very hot water with 2 tablespoons of the granulated sugar and stir until the sugar is dissolved. Let the sugar syrup cool to room temperature.
In a small saucepan, combine the remaining 1/2 cup of granulated sugar and 1/4 teaspoon of cream of tartar with 3 tablespoons of water and cook over moderately high heat, washing down the sides of the pan with a wet pastry brush and gently swirling the pan, until a deep amber caramel forms, about 6 minutes. Remove from the heat. Add 1/4 cup plus 2 tablespoons of the heavy cream and the butter and stir just until combined. Cook over high heat for 1 minute. Transfer the caramel sauce to a heatproof bowl and let cool completely.
In a large bowl, using an electric mixer, combine the remaining 1 cup of heavy cream with 1/4 cup of the confectioners' sugar and 1/2 teaspoon of the vanilla. Beat in the sour cream and salt, then beat until firm peaks form. Fold in the cooled caramel sauce and refrigerate.
Peel the parchment paper off both cakes and brush the surfaces with the sugar syrup. Spread the caramel cream over each cake in a thin even layer. Working with the long side nearest you, roll up each cake, using the wax paper to help you form a tight roll. Wrap the rolls in the wax paper, transfer to a baking sheet and freeze until very firm, about 2 hours.
Melt the chopped chocolate in a medium microwave-safe bowl. Whisk in the oil. Let the glaze cool slightly. Line a baking sheet with wax paper. Unwrap the rolls and cut each one into 5 pieces. Working quickly, dip both ends of each roll into the chocolate glaze, then dip the tops and bottoms, letting any excess glaze drip back into the bowl. Place the rolls on the prepared baking sheet and refrigerate just until the glaze is completely set. Serve the yodels cold or at room temperature.