Prepare Chocolate Layer: In a saucepan, combine 2 cups full-fat milk, 1/2 cup starch, 1/4 cup sugar, and 2 tablespoons unsweetened cocoa powder. Whisk until no lumps remain. Heat on slightly higher than medium heat, stirring constantly to prevent lumps and sticking. When mixture thickens and steam rises, reduce heat and continue stirring. Add 1 tablespoon Nutella and stir until fully blended and smooth. Remove from heat.
Assemble First Layer: Pour the chocolate mixture into a ceramic or Pyrex mold, spreading evenly to cover the bottom. Insert tea biscuit pieces spaced apart over the chocolate layer, leaving gaps between biscuits.
Prepare Vanilla Layer: In another saucepan, combine 2 cups full-fat milk, 1/2 cup starch, and 1/4 cup sugar. Whisk until starch dissolves completely. Heat on slightly higher than medium heat, stirring constantly until mixture thickens. Add 1 vanilla sachet and stir until fully incorporated. Remove from heat.
Assemble Second Layer: Immediately pour the hot vanilla mixture evenly over the biscuits, ensuring it fills the gaps and covers the sides. Sprinkle cocoa powder evenly over the surface.
Add Topping and Chill: Finely chop 100 grams white chocolate and sprinkle it over the cocoa powder layer. Cover the mold and refrigerate overnight to allow flavors to meld and cake to set.
Serve: Cut into pieces and serve chilled. The cake has a smooth texture with rich chocolate, Nutella, and vanilla flavors.