1.Preheat oven to 180°C. Grease a deep 20cm round cake pan; line base and sides with three layers of baking paper, extending the paper 5cm over side.
2.Beat butter, paste and sugar in a medium bowl with an electric mixer until pale and fluffy. Beat in eggs, one at a time. Stir in sifted flours and milk, in two batches. Stir in chopped biscuits. Spread mixture into pan; smooth surface.
3.Bake for 1 hour or until a skewer inserted into the centre comes out clean. Stand cake in pan for 5 minutes before turning, top-side up, onto a wire rack to cool.
4.Meanwhile, make crème pâtissière: Heat milk and paste in a small saucepan over medium heat until almost boiling. Whisk egg yolks, sugar and cornflour in a medium bowl; gradually whisk in hot milk mixture. Return mixture to pan; whisk over low heat for 5 minutes or until mixture boils and thickens. Remove from heat; stir in 40g of the butter. Reserve a third of the mixture in a small bowl. Stir chocolate and remaining butter into the remaining mixture until melted through. Cover bowls; refrigerate until cold.
5.Split cake into three layers. Whisk each crème pâtissière mixture until smooth. Place the bottom cake layer on a cake stand or plate; spread with half the chocolate crème pâtissière. Top with middle cake layer; spread with plain crème pâtissière. Finish with top cake layer and remaining chocolate crème pâtissière.
6.Decorate with extra chopped biscuits.
The cake and crème pâtissière can be made a day ahead. Store cake in an airtight container at room temperature; refrigerate crème pâtissière, covered tightly.Note