Lay out plastic wrap in an 8″ (20 cm) diameter form, cover it well and have the edges of wrap spill out of the form, to make it easier to take the cake out later.
Make the cream: put 5 tbsp sugar to caramelize. Pour in 1 cup hot water to the resulting caramel. Be careful because it sprays.
Put it to boil and once the caramel has melted in the water, pour in the milk (3 cups). In the meantime, beat the egg with the flour and 1 cup milk. Pour the resulting sludge in a trickle to the caramelized milk, while stirring nonstop. Add the butter, packet vanilla and rum essence.
Boil the cream until thickened, while stirring nonstop. If it's runny you can add more flour. Pour some of the cream into the form in which you'll be baking the cake. Put pieces of biscuits, dipped in warm milk. Then place another layer of cream and biscuits, until you've used up all of the products.
Put the cake to harden in the fridge for a few hours (make the glaze the next day and decorate the cake). Carefully invert the cake into a cake plate, remove the plastic wrap.
To make the glaze, put the butter, 1/4 cup water, sugar, vanilla packet and 1 tbsp cocoa in a pot and leave it to boil.
Separately, mix the flour with the other 1/4 cup water and pour this into the boiling cocoa, stirring nonstop with a whisk until the glaze thickens and becomes shiny.
Remove from the stove and let the glaze cool, stirring occasionally to prevent a crust from forming. Smear the cake with the glaze and decorate with crumbled or grated biscuits.