Blitz the ground hazelnuts in a food processor along with all the other ingredients up to and including the maple syrup until the mixture holds together. Divide the mixture into approx. 15 portions, shape into balls, press 1 hazelnut into the centre of each praline. Cover and refrigerate for approx. 1 hr.
Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie; the bowl must not touch the water. Melt the chocolate and stir until smooth. Dip the pralines in the chocolate, leave to drip briefly on a rack, toss in pistachios or hazelnuts, or dust with cocoa powder. Transfer the pralines to a sheet of baking paper, cover and chill for approx. 1 hr.