You dont say what grinder you are using.
Single shot baskets are a pain.
I dont think there is enough depth of grinds for the espresso to work properly.
I gave up long ago and most replies on the subject sort of say the same.
Why use a single basket?
Why not make a short double shot (?ristretto) using the double basket (with a few less grinds if you want with a slightly finer grind). *Pull the PF away early (start with 20ml max). *The colour should be consistently very dark. *This seems to be the standard at good coffee shops in Perth. *Giotto is great machine. *I find it is very sensitive to the grind.
This is bound to attract a reply!