Preheat oven to 180ºC and line 2 trays with baking paper.
Beat together the butter and Natvia until light and fluffy, then add vanilla and combine.
Sift in plain flour and cornflour and fold into butter mixture. Add the chocolate Fangks and stir well to combine.
Scoop out tablespoon sized balls of the mixture and form into balls with your hands. Make sure to make an even number of balls! Use a fork to flatten slightly and bake for 15-20 minutes or until firm. Remove onto a rack to cool completely.
For the filling, beat together butter and Natvia until pale and creamy. Pour in the cooled melted chocolate and continue to beat until smooth.
Spoon the mixture into a piping bag and pipe over half the biscuits. Sandwich together with the remaining biscuits.