Stir chocolate in large bowl set over saucepan of boiling water until melted and smooth, making sure that water does not touch bowl and that the chocolate does not get too hot. Remove from over water.
Dip into melted chocolate to cover half the Melba snack. Gently shake off excess chocolate.
Place on baking sheet and quickly sprinkle with chopped pistachios. Refrigerate until chocolate is firm, about 35 minutes.
