In a large bowl, sift together the flour, salt, and baking powder then set aside.
Combine the butter and granulated sugar in the bowl of an electric mixer fitted with paddle attachment if you have one and cream on high speed until fluffy if you do not have an electric mixer just beat rapidly for around 10 minutes to achieve the same outcome.
Beat in the eggs.
Lower speed if using electric mixer and add the flour mixture in two additions, mixing until well combined.
Add and mix in the vanilla.
Divide dough in half, and cover with cling film then chill for 30 minutes.
Preheat the oven to 180°C / 350°F / Gas mark 4.
Line 2 baking trays with greaseproof paper and set aside.
Roll out 1 piece of dough on a lightly floured work surface.
With a cookie cutter, cut out rounds or use a heart shaped one for several cut outs to make the look of the shamrock come to life.
Working with 1 round at a time, sprinkle the surface with green crystal sugar.
Arrange the biscuit shapes on the baking tray and chill until firm, about 15 minutes, or until ready to bake.
Repeat process with the remaining dough.
Bake until edges just start to brown about 10 to 12 minutes.
Let cool completely on the baking tray or wire racks.