Step 1: Preheat your oven to 375°F (190°C). Line a 10x15-inch (25x38 cm) jelly roll pan with parchment paper, extending it slightly over the sides for easy lifting, and lightly grease the parchment.
Step 2: In a medium bowl, whisk together the all-purpose flour and unsweetened cocoa powder until thoroughly combined and free of lumps. Set this dry mixture aside.
Step 3: In a large mixing bowl, using an electric mixer, beat the egg yolks with half of the granulated sugar (¼ cup or 50g) on medium-high speed. Continue beating for 3-5 minutes until the mixture is light, thick, and pale yellow. Stir in the vanilla extract.
Step 4: In a separate, very clean, large mixing bowl, beat the egg whites with the cream of tartar (if using) on medium speed until soft peaks form. Gradually add the remaining granulated sugar (¼ cup or 50g), increasing the speed to high, and beat until stiff, glossy peaks form. Be careful not to overbeat, as this can make them dry.
Step 5: Gently fold one-third of the stiff egg whites into the egg yolk mixture to lighten it, then carefully fold in the remaining egg whites until just combined. Use a rubber spatula and a gentle hand to avoid deflating the airy mixture.
Step 6: Sift the reserved flour and cocoa mixture over the egg mixture. Gently fold until no streaks of flour remain, again being careful not to deflate the batter. Pour the batter evenly into the prepared pan and spread it gently to the edges.
Step 7: Bake for 12-15 minutes, or until the cake springs back when lightly touched in the center and the edges are set. Overbaking will result in a dry cake that is difficult to roll, so keep a close eye on it.
Step 8: While the cake is still warm, carefully invert it onto a clean kitchen towel heavily dusted with cocoa powder (or powdered sugar). Peel off the parchment paper. Trim any crisp edges. Starting from one short side, tightly roll the warm cake and towel together. Let it cool completely on a wire rack, seam-side down.
Step 9: In a medium saucepan, combine ½ cup (100g) granulated sugar and ¼ cup (60ml) water. Cook over medium heat, stirring until the sugar dissolves. Increase heat and boil without stirring until the caramel turns a deep amber color. Watch carefully to avoid burning.
Step 10: Carefully remove the caramel from heat and slowly whisk in the heavy cream (it will bubble vigorously). Stir in the butter and salt until smooth. Let the caramel cool completely, then refrigerate until well chilled and slightly thickened. This can take an hour or more.
Step 11: In a large bowl, beat the softened cream cheese until smooth and creamy. Gradually beat in the chilled caramel sauce until fully incorporated and the mixture is light and fluffy. Fold in 1 cup of the roughly chopped roasted peanuts.
Step 12: Carefully unroll the cooled chocolate sponge cake. Spread the caramel cream filling evenly over the entire surface of the cake, leaving a ½-inch border on all sides. Sprinkle the remaining ½ cup of chopped peanuts evenly over the caramel cream.
Step 13: Starting from the short end you originally rolled, gently and tightly roll the cake back up, without the towel this time. Be firm but gentle to prevent cracking and ensure a tight roll.
Step 14: Transfer the roulade to a serving platter, seam-side down. Cover loosely with plastic wrap and refrigerate for at least 2 hours, or preferably overnight, to allow the roulade to set and the flavors to meld.
Step 15: Before serving, dust generously with unsweetened cocoa powder or powdered sugar, and if desired, drizzle with extra caramel sauce and a few whole peanuts for garnish. Slice with a sharp, warm knife for the cleanest cuts, wiping the knife between slices.