In a medium-sized bowl, whisk together the instant vanilla pudding mix and 3 cups of cold milk. Whisk until the pudding thickens, about 2-3 minutes. Set aside 1 cup for the chocolate mixture and use the rest for the layers.

In a square baking dish, spread a small amount of vanilla pudding on the bottom, and then arrange a single layer of digestive biscuits or graham crackers. You may need to break some biscuits to fit evenly across the dish.

Spread half of the vanilla pudding evenly over the layer of biscuits, making sure the surface is smooth.

Add another layer of biscuits over the pudding, followed by the other half of the vanilla pudding. Finally, top with a third layer of biscuits to finish the layers.

In a small saucepan, heat the heavy cream over medium-low heat until it just begins to simmer. Remove from heat and add the chopped bitter chocolate. Stir until the chocolate melts completely and is slightly cold. Mix with the remaining vanilla pudding.

Pour the chocolate mixture over the top layer of biscuits, spreading it evenly to cover the entire surface.

Cover the cake with plastic wrap and refrigerate for at least 4 hours, or overnight, to allow the biscuits to soften and the flavors to meld together.

Once chilled, slice the cake into squares and serve cold. Enjoy your easy, no-bake Eclair Cake!