Mix the whites with the mascarpone base bit by bit being very careful not to take the air out from the whites. Mix the coffee with some water and some sugar. Put a generous spoonful of mascarpone mix on the bottom of the cake container and level it off. Dip the sponge fingers in the coffee and lay them next to each other. Cover them with half of the mascarpone mix and do a second layer of biscuits then cover with the other half of the mix.