Whisk sugar, cornstarch, cocoa, and salt in a saucepan. Stir in half the milk until smooth, then add remaining milk and egg yolks. Cook over medium heat, whisking constantly, until thick and bubbly, about 6 to 8 minutes. Off the heat, stir in butter and vanilla. Cool 5 minutes.

Arrange a single layer of biscuits in a greased 20 cm square dish. Spread one-third of the custard over them. Repeat two more times, ending with custard on top.

Cover custard surface with plastic wrap and refrigerate at least 4 hours until firm. Slice into squares, plate, and top with whipped cream and chocolate curls.