In a medium saucepan, whisk together the granulated sugar and cornstarch.

In a separate bowl, whisk the egg yolks with the milk until smooth.

Slowly pour the milk mixture into the saucepan in several additions, whisking well after each addition.

Place the saucepan over medium-low heat and cook, whisking constantly, until the custard thickens and begins to boil, about 6–8 minutes.

Remove from the heat and stir in the butter and vanilla extract until completely melted and smooth.

Transfer the custard to a heatproof bowl and cover the surface directly with plastic wrap to prevent a skin from forming.

Cool completely to room temperature.

Fold in the whipped topping, if using, then refrigerate until ready to assemble.

Pour the ¼ cup milk into a shallow bowl.

Quickly dip each digestive biscuit into the milk for about 1 second per side.

Arrange one layer of soaked biscuits in a 9×9-inch square baking dish.

Spread half of the chilled vanilla custard evenly over the biscuits.

Repeat with another layer of dipped biscuits and the remaining custard.

Finish with a final layer of dipped biscuits.

Cover and refrigerate for at least 4 hours to allow the Vanilla No Bake Biscuit Cake to firm up.

Combine the chopped chocolate and milk in a heatproof bowl.

Melt over a double boiler or microwave in 30-second intervals, stirring until smooth.

Pour the chocolate mixture over the chilled biscuit cake.

Spread evenly with a spatula.

Refrigerate for another 20–30 minutes until the chocolate is fully set.

Slice into squares and serve chilled.