For the vanilla custard layer: In a medium saucepan, add in your sugar and cornstarch. Whisk together to combine, and set aside. In a medium bowl, combine your egg yolks and milk. Whisk together until the egg yolks are broken up and incorporated. Gradually add your milk/egg mixture to the cornstarch and sugar (in 3 or 4 additions), whisking well after each addition.
Place your pot over a stove burner set to medium-low heat. Cook your custard, whisking constantly, until it thickens and starts to boil (about 6 to 8 minutes). Remove from the heat and add in your butter and vanilla. Whisk together until the butter is melted and everything is combined.
Transfer the cooked custard to a medium, heat-proof bowl. Cover the custard with plastic wrap touching the surface of the custard (to prevent skin formation) and let it cook to room temperature completely. Once cooled, fold in your whipped topping and refrigerate until ready to use.
Assembling the cookies and custard layers: Measure out ½ a cup of milk into a medium-sized bowl (large enough to fit your biscuits). Quickly dip your digestive biscuit into milk for about 1 second on each side. Arrange your biscuits into one layer in a 9x9 inch square dish.
Top the biscuit layer with half of your vanilla custard. Arrange another layer of biscuits on top of the custard layer by again dipping the biscuits in milk and placing in your square dish. Top with the remaining half of the custard. Finish off the layering with one more layer of milk dipped biscuits. Cover and refrigerate for at least 4 hours to let the custard set.
Make the chocolate topping: In a medium sized, heat proof bowl, combine your chocolate and milk. Melt the chocolate over a bain-marie or in your microwave set to high for 30 seconds. Stir your chocolate occasionally, and continue heating in the bain-marie or in the microwave in 10 second intervals until melted and smooth.
Pour your melted chocolate/milk mixture over your biscuit cake. Spread into an even layer to cover the top. Place in the fridge for another 20 to 30 minutes to let the chocolate layer set. Slice and serve!