Melt 1 cup of chocolate chips over low heat or in the microwave. Cream butter and sugar then add melted chocolate chips and vanilla. Beat in eggs. Stir in the flour mixture and remaining chocolate chips.
Wrap the dough in plastic wrap and freeze until firm (about 20 minutes). Preheat oven to 180 degrees C.
Form dough into 3cm balls, then roll the balls in icing sugar. Place on ungreased baking trays and bake 10-12 minutes. Cool on a wire rack.