First make the crust. Begin by melting butter in medium saucepan over low heat. Add 60g chocolate to the butter; stir until chocolate melts and remove from heat. Whisk in 1/3 cup flour, 1/3 cup sugar, 1 egg, 1/2 teaspoon vanilla essence and pinch of salt. Spread batter in thin layer on bottom of greased and floured 20cm square tin.
Bake at 180 degrees C until a skewer inserted into centre comes out with only moist crumbs attached, about 10 minutes. Cool.
Next make the filling. In a small saucepan, stir cream and coffee powder over medium-high heat until coffee powder dissolves. Cool.
Beat cream cheese and 2/3 cup sugar in large bowl until light and fluffy. Beat in 1 egg and yolk. Add 1/4 cup sour cream, 1 tablespoon flour, 1/2 teaspoon vanilla and the coffee/cream mixture; beat until smooth. Stir in chocolate chips. Spoon filling evenly over crust.
Reduce oven temperature to 165 degrees C. Bake about 35 minutes, or until filling puffs slightly around the edges and centre is set. Cool completely on rack. Cover pan with foil and chill overnight.
Now make the topping. In a small saucepan, stir 1 cup sour cream and icing sugar over low heat until sugar dissolves; do not boil. Spread topping evenly over filling. Cool slightly. Cover and chill until topping sets, at least 2 hours. Cut into 24 bars. Serve cold.