Place the milk, butter and sugar into a saucepan on a stovetop, on a medium heat. Stir until the sugar dissolves.
In the meantime, beat the egg yolks, then add the condensed milk, and beat for a further 10 seconds. Then add the cocoa powder and cornflour, and beat until well combined.
Once the milk mixture comes to a simmer on the stove, slowly add the egg mixture, whisking until combined and the mixture thickens (roughly 2 - 3 minutes).
Remove from the stove, transfer into a glass bowl and place in the freezer to cool.
Place the whipping cream in a bowl, add the sugar, and beat until stiff.
Once the mixture in your freezer has cooled, add the whipped cream to it by slowly folding it in.
Place the chocolate crème into a piping bag with a round tip.