Spreading and smoothening foods on Shabbos:
Does not contain a smoothening prohibition: It is permitted to smoothen foods on Shabbos and doing so does not contain the Smoothening prohibition.
The reason for this is because it is anyways possible to eat the food in its current state [without having smoothened it].
Smoothening a food in order to beautify it: Nevertheless one who is stringent with regards to [not smoothening] apple dishes [or potatoes] and foods of the like which are commonly smoothened [in order to better their appearance] will be blessed. [However according to other opinions this is forbidden from the letter of the law due to the building prohibition.]
It is permitted to spread and smear foods on Shabbos if one does not intend to do so in order beautify the food by doing so. If one does intend to beautify the food through spreading it then it is proper to be avoided if it is common to spread this food. [if it is not common to spread the food it is always permitted to spread it.]
If the food is unable to be eaten unless one spreads the food on it, may it nevertheless be spread on it?
No. If the food cannot be eaten without the spread then spreading a food on it contains a Rabbinical “Smoothening” prohibition, and is forbidden to be done even right before the meal.
May one smoothen the top of a dip such as Chumus, Tehina, mashed potatoes?
If one is doing so in order so the dip looks fancy then one is to be stringent to avoid doing so.
May one smoothen and spread cooked apples onto bread?
May one spread butter onto his bread?
May one smear icing onto cake on Shabbos?
Yes. One may smear icing throughout the entire cake in order to sweeten it. However one is to refrain from further spreading the icing in order to make the cake look fancy.
May one spread cream on a cake using an icing tip to form designs?
 Rama 321/19
 M”B in Biur Halacha “Bemachal”. Ketzos Hashulchan 146 footnote 11.
 Ketzos Hashulchan 146 footnote 11 learns that the “one who is stringent” only applies to commonly spread foods. However those foods which are not commonly spread are permitted in all circumstances to be spread. See also Shabbos Kehalacha Vol. 3 20/59
 M”B in Biur Halacha “Bemachal” , Ketzos Hashulchan 146 footnote 11.
 The reason for this is because when spread to decorate the food then it resembles more the smoothening prohibition which is learned from the smoothening of the skins of animals which is done in order to beautify the skin. [Biur Halacha]
 Kitzur Shulchan Aruch 80/25 who learns this from the Magen Avraham 340/17 which rules that pressing figs together and smoothening them down to beautify their appearance involves the Building prohibition. Other Poskim however [Cheishev Haeiphod 2/77; Daas Torah 321, and so is apparently the opinion of the M”B and Ketzos Hashulchan [146 p. 12] which do not mention this prohibition in their summary and commentary of the above Halacha in the Rama] rule that this prohibition of building stated by the Magen Avraham only applies by pressing hard items together and making them into one unit. However to spread something nicely does not resemble in any way the building prohibition but is rather only proper to avoid due to the smoothening prohibition as rules the Rama.
 Ketzos Hashulchan 146 footnote 11; Shabbos Kehalacha Vol. 3 20/59
 Magen Avraham 321/29, brought in M”B 321/81; Shabbos Kehalacha Vol. 3 20/59
 So rules Peri Megadim 321/29
 Shabbos Kehalacha Vol. 3 20/61
 Magen Avraham 321/29, brought in M”B 321/82 and Ketzos Hashulchan ibid
 M”B 321/82
 So rules Ketzos Hashulchan 146 footnote 12, as if done to look fancier then since this is commonly done in places it is proper for one to be stringent, as stated above that foods which are commonly spread for fanciness one who is stringent to refrain from doing so is blessed, as it appears like smoothening.
 Piskeiy Teshuvos 321/15
 As one is not smearing the icing using a knife and hence how is it similar to Mimacheik or Mimareiach? Is squeezing cream from a tube considered Mimacheik? No, hence the allowance to dab cream for pleasure on one’s body.