In a saucepan, whisk sugar and cornstarch together.

In a bowl, whisk egg yolks and milk until smooth.

Gradually add the milk mixture into the dry ingredients, whisking well.

Cook over medium-low heat, whisking constantly until thickened and gently boiling (6–8 minutes).

Remove from heat, then stir in butter and vanilla until smooth.

Transfer custard to a bowl, cover with plastic wrap touching the surface, and let cool completely.

Fold in whipped topping (if using) and refrigerate.

Dip biscuits briefly in milk (1 second per side).

Arrange a layer in a 9x9-inch dish.

Spread half of the custard over the biscuits.

Add another biscuit layer, then remaining custard.

Finish with a final biscuit layer.

Refrigerate for at least 4 hours to set.

Melt chocolate with milk using microwave or bain-marie.

Pour over the cake and spread evenly.

Chill for another 20–30 minutes until set.

Slice and serve your Vanilla No Bake Biscuit Cake.