Mix the mascarpone, capers and chives, lemon zest and enough lemon juice to flavour it, but not enough to make it runny. Season really well.
Spread the mascarpone mix over all the bread slices, then cover completely with a layer of salmon. Put one bread slice on top of another, so you have a four separate stacks, each 2 deep (with smoked salmon on top).
Carefully cut each into 6 bite-sized squares with a serrated knife. Dip the ends of the squares in chives and decorate with extra shreds of lemon zest if you like.