Preheat oven to 170°C. Lightly grease 45cm by 30cm Swiss-roll pan. Line with greaseproof paper, with overhang on 2 sides; grease paper.
In a food processor, pulse peanuts and ¼ cup granulated sugar until finely chopped. Remove ½ cup peanuts; set aside. Continue pulsing remaining peanuts until finely ground. Add cocoa powder, flour and ¼ teaspoon salt; pulse until just blended.
In a large bowl, with mixer on medium-high seepd, beat eggs, brown sugar and remaining ¼ cup granulated sugar until mixture is tripled in volume and very thick and pale, and batter forms soft peaks when beaters are lifted. Gently fold in cocoa mixture, being careful not to deflate eggs.
Gently spread batter in an even layer in prepared pan. Bake for 17 to 19 minutes or until centre springs back when lightly pressed. Using greaseproof paper overhang, quickly and carefully slide cake off pan and onto wire rack to cool completely.
While cake cools, place chocolate in a heatproof medium bowl. In a medium saucepan, heat cream on medium until bubbles form around edge. Pour over chocolate. Let stand for 1 minute, then stir until smooth. Place 1 tablespoon mixture in small resealable plastic bag.
In a large bowl, with mixer on medium speed, beat butter, ½ cup peanut butter and pinch salt until smooth and creamy. With mixer running, gradually add melted-chocolate mixture and then vanilla. Beat until blended. Refrigerate mixture for 5 minutes or until it’s the consistency of icing.
Lightly dust top of cake with cocoa. Invert cake onto a large cutting board lined with wax paper; discard greaseproof paper. Cut cake to form four 10cm-wide rectangles. Place 1 rectangle on serving platter. Spread ¼ icing evenly over cake, sprinkle with ⅓ reserved chopped peanuts, and top with another rectangle; repeat twice. Spread remaining icing all over cake.
Place remaining tablespoons peanut butter in small resealable plastic bag. Microwave on high for 10 seconds or until runny. Snip small hole in corner and pipe decorative pattern over cake. Snip small hole in corner of bag with reserved chocolate mixture. Decorate with chocolate. Let it stand for at least 1 hour and up to 2 hours, or refrigerate for up to 2 days. Bring cake to room temperature before serving.