Combine the chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water. Stir until just melted, about 6 minutes; remove from the heat and whisk until smooth and shiny.
Fill a large bowl halfway with ice water. Combine 2 egg yolks, 6 tablespoons sugar, the cognac, vanilla and 2 tablespoons hot water in a separate heatproof bowl. Beat with a mixer until thick and pale yellow, about 7 minutes. Place the bowl over the pan of simmering water and continue beating until warm and tripled in volume, about 8 more minutes. Carefully rest the bowl in the ice water and beat until the mixture is cool and thick, about 2 more minutes. Gradually beat in the melted chocolate mixture until incorporated.
Wash and dry the beaters. In a separate large bowl, beat 2 egg whites, the remaining 1 teaspoon sugar and the salt until stiff peaks form, about 2 minutes. Stir one-quarter of the egg white mixture into the chocolate mixture, then gently fold in the rest until incorporated. Transfer to four 8-ounce ramekins. Cover and refrigerate until set, at least 4 hours or overnight.
Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper. Using a 1 1/2-inch cookie cutter or small knife, cut out 4 hearts (or circles) from the pastry. Brush with the beaten egg, transfer to the prepared baking sheet and freeze until firm, about 5 minutes. Bake until puffed and golden, 10 to 12 minutes. Let cool completely. Top each ramekin with whipped cream and a puff pastry heart.