One may not write letters or draw pictures on a cake using candies or frosting. Though this is not prohibited by Torah law since the letters are not long lasting, it is rabbinically prohibited.
If a cake has letters or pictures on it, one may not cut through the letters, but only between them. If the letters are formed in the cake or cookie itself, as is typically found on tea biscuits, they may be cut, as these letters are insignificant (below 18:3).
If it is necessary to open a package of food that has writing on it, one should try to do so without tearing the letters. However, if there is no way around this, he may open the bag even if he will tear letters (as explained below in 18:3).
One may not churn milk into butter on Shabbat. Many believe this is prohibited by Torah law as a tolada of Boneh, because it involves changing the food from a liquid to a solid (Shabbat 95a; MT 10:13). One also may not shape food into impressive designs, as this is a type of building (MA 340:17; Ĥayei Adam 39:1).
One may spread food on bread or crackers, as the prohibition of spreading (Memare’aĥ) does not apply to foods. One also may change the appearance of the spread to make it look more appealing, smoothing it as desired. Thus, one may put hummus on a serving plate and spread it into a circle for aesthetic reasons since the food is already ready to eat, and smoothing it does not improve it in any way. Some are stringent and do not permit smoothing foods to make them look more appealing. One who chooses to be stringent should be commended (Rema 321:19).