In a mixing bowl, combine and mix all the dry ingredients for the cookie crust. Next, add in the softened butter and vanilla and mix it in with a spatula or your hands till the mixture resembles coarse crumbs. Pour the mixture into any greased medium sized square or rectangle baking pan depending on how think you want the bars to be (I used a 59 inch rectangle pan). Press the mixture into the pan to form an even layer and with a fork prick tiny holes on the dough.
Bake the crust in a preheated oven at 300 degrees fahrenheit for about 20 minutes on the center rack until the dough is partially cooked and firm.
While the crust bakes, make the filling by combining the egg yolks and sugar and whisking to combine. Next, add in the lemongrass tea, lemon juice and salt and whisk again thoroughly. To finish, add in the flour, cornstarch and finally food colouring (if using) and give it a final whisk till the mixture is smooth and there are no more lumps.
Take out the crust from the oven when it is done. Pour over it the filling and bake again at the same temperature on the center rack for 15 more minutes and then bake an additional 10 minutes on the top rack.
Take the bars out of the oven when done and let it cool completely. After, freeze in the pan for about 1 hour till the filling is firm.
After freezing, take it out and run a knife around the sides to loosen it up and take it out of the pan. Cut into 8 equal pieces and finish with a dusting of powdered sugar.
The bars will keep well in the refrigerator for up to 2 days and in the freezer in an airtight container for up to 2 weeks. Happy Baking!
