First add the corn starch to a large mixing bowl, add 3 Cups of water and mix well until the corn starch is dissolved. You will have a corn flour slurry now.
Heat a pan in medium flame and add the remaining water and sugar. Stir well until the sugar is dissolved. Add the lemon extract, mix well and continue cooking until the water starts to boil.
Once the water comes to a rolling boil, add the cornflour mixture to the pan. Mix well. Turn the flame to low-medium.
Now, add the the food color and stir continuously. The mixture will start to thicken.
Once the mixture is thick and sticky, start adding the ghee. Add 1 Tbsp at a time, mix well until the corn mixture absorb the ghee. Then add the next Tbsp of ghee. Keep cooking until the ghee is no more absorbed by the halwa. The halwa should now look glossy, transparent and does not stick to sides of the pan. You can use the rest of the ghee for some other recipe.
Now, turn off the flame, add the cardamom powder and chopped nut. Mix well.
Transfer the mixture to a greased pan / plate. I used a 9X9 baking pan. Leave it to cool completely for 2 to 3 hrs to room temperature
After the halwa is set, use a sharp knife to loosen the edges and transfer the halwa to a plate. Cut it into cubes.
Enjoy.