This is easier than it looks and needs to be made one day ahead, which is good for those of us who like to be prepared.
Make a chocolate tart case, using a sweet shortcrust pastry recipe with 2 level tablespoons of sieved cocoa in place of two tablespoons of the flour. Roll the pastry as thin as you dare for a professional look. Blind bake the pastry case. While it is cooling make the filling. Chop 300g hazelnut chocolate (one with smooth or finely chopped hazelnuts, not whole nuts) and place in a bowl with 300ml double cream. Melt together on lowest possible heat (a microwave is good, in 1 minute bursts on its lowest setting). Stir the filling until smooth, cool to room temperature and pour into the pastry base. Chill overnight to set. Serve dusted with cocoa powder and a spoonful of boozy cherries.