Now here's what we need:
1 packet of Milk biscuits (I used milk bikis)
1 cup powdered sugar
1 cup full cream milk
1 tbsp all purpose flour/cornstarch
4 tbsp bitter cocoa powder
50 gms low salt butter (You may use unsalted butter and add a pinch of salt)
Prepare the chocolate roux by mixing sugar, cocoa powder, all purpose flour in half a cup of warm milk. If you dislike using all purpose flour/maida use cornstarch instead. Melt butter and add the above dry mixture and heat on medium-low flame. Add a more milk or cream (to make it richer) to the above if required. Mix and fold in thoroughly to get a smooth velvety mixture. Chocolate roux should be ready once it begins to stick stiffly to the back of the spatula. Remove from the gas and allow to cool.
Prepare warm chocolate milk by mixing a tsp of sugar and cocoa powder to 1/2 cup of milk. Dip each milk biscuit in Warm chocolate milk and place them on an aluminum foil side by side depending on the size of the cake you want. Spread the roux evenly leaving about an inch of space in the edges. Place another layer of chocolate milk dipped biscuits on the roux and press down gently. Spread another layer of choco roux followed by biscuits and roux again. Make the height of cake not more than 5-6 biscuits as it will not stand well beyond that. Pour the remaining roux on top of the cake and dress them with chocolate chips / grated chocolates as desired.
Cover with an aluminum foil and refrigerate for 3-4 hours. Remove from the fridge and cut across the biscuits to get beautiful milk biscuit cake slices.
If you want crispier biscuit cakes, then use Marie biscuits instead of milk biscuits. Cut diagonally and you will get beautifully textured cake.