Heat the oven to 320°F (160 °C).
Beat the egg with the sugar until whitened. Add the oil, milk, mixed together flour and baking powder, ground walnuts. Beat with a mixer after adding each subsequent ingredient. Pour the result into an 8″ (20 cm) form with removable sides. Bake the layer in the preheated oven. Once baked, leave it to cool in the form.
For the Cream:
Put 3 cups (700 ml) milk on the stove to heat up. Dissolve the 3 puddings and vanilla sugar in the remaining milk. Once the milk is boiling, add the pudding and stir until the cream comes to a boil. At that point, remove from the stove.
Leave it to cool, stirring periodically to prevent a crust from forming.
Beat the soft butter with a mixer and slowly add the now cooled cream to it.
Beat to obtain a fluffy, smooth cream. Pour it over the cake layer in the form.
Leave for 2 hours in the fridge. Remove the cake ring and decorate.