Q 1. Re question 1331, the Rav permits icing a cake…with whipped cream from ..an aerosol can….I read somewhere that using such a can on Shabbos was forbidden due to the melacha of molid. Can the Rav please clarify?

A. I’m quoting answer 403 “The main issue with using canned sweetened whipped cream propelled by nitrous oxide, for decorating desserts, ice creams and cakes on Shabbos, is the prohibition of molid or creating something new (Shabbos 51b). Inside the can the cream is in a compressed liquid state, when sprayed it changes temporarily, for a few minutes, to firmer self-standing foam.
Some Poiskim maintain it should not be used in Shabbos (Hilchos Shabbos Bamitbach p.961 in the name of Horav M. Klein and Horav M. Shtern Zt”l, 39 Melochos 2 p. 371). Some however permit its use. (Rivavos Ephraim O.H. 8; 158: 11, Orchos Shabbos 15: 45, Hemdat Yamim, Balak 5771)
Horav Shlomo Miller’s Shlit”a opinion is that when needed you are allowed to use it on Shabbos.

Q 2. What does when needed mean? Obviously one can make it thru Shabbos without whipped cream. If a dessert looks nicer with whipped cream on top is that called when needed?? Is there any problem with the fact that it comes out in a star  shape?

A. Horav Shlomo Miller Shlit’a explained, that it is needed for a” tzorech mitzva” such as honoring a guest, something you may not do on a regular Shabbos.
The star shaped extraction is of no consequence, since it is not intended or used for creating or transmitting a special message. (See question 1331). It is also arguable that the star shape (multiple angles) is mainly done to maintain the stability of the sprayed foam, so it will not collapse and fall flat on the surface it was sprayed on.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a