Whisk egg yolks, sugar, and vanilla extract in a saucepan off heat until pale and smooth.
Add sifted cocoa powder and cornflour, whisking until fully combined.
Gradually whisk in the milk and cream until smooth.
Heat the mixture over medium heat, stirring constantly to prevent lumps and scraping the pan’s bottom.
Cook for 5-8 minutes until the mixture thickens to a custard consistency, then remove from heat immediately.
Stir in chopped dark chocolate and a pinch of salt until fully melted and silky smooth; strain if desired for extra smoothness.
Divide the pudding into serving glasses or ramekins and cover directly with plastic wrap to avoid skin formation.
Refrigerate for at least 3 hours until the pudding is set.
Whip cold heavy cream with sugar and vanilla extract until soft peaks form.
Top each chilled pudding pot with whipped cream.
Sprinkle finely chopped dark chocolate over the whipped cream and serve.