2) For the Dough: In a medium bowl using a hand held electric mixer, beat together the butter and sugar until smooth. Add the citrus rind and continue to beat until light and fluffy, about 2 mins. Add the yolks, one at a time, beating well after each addition. Beat in the extracts.
3) Sift the flour, baking soda and salt together in a bowl. While mixing at a low speed, add flour mixture to the creamed ingredients to make a soft dough.
4) Using a small ice cream scoop or measuring spoon scoop heaped tablespoons onto the prepared trays about 5cm apart. Freeze for at least 30 mins. (The biscuits can be frozen for up to 1 month.)
5) Bake the biscuits, straight from the freezer, until edges are firm and bottoms are lightly browned, 15 to 17 mins. Transfer to a rack to cool.
6) For the icing: Mix the icing sugar, colored sugar and appropriate flavored zest in a medium bowl. Add the citrus juice and mix with an electric mixer to make a firm but pourable icing. (If needed, add up to 1 tsp more juice, but keep in mind that if the icing is too loose it won't set.)
7) Dip the rounded side of the cooled biscuits into the icing; then let the excess icing fall back into the bowl. Dry cookies icing side up on a rack. Serve or store in an airtight container for up to 4 days.