I do not believe that food should ever be used as a punishment, dessert or otherwise. It has been said that it can lead to eating disorders when the child gets older. When my daughter was in Kindergarten, a teacher once did not let my daughter eat cupcakes with the rest of the class as a punishment. My daughter did develop an eating disorder when she got older. I cannot say for sure if it is related to the cupcake incident, but I will forever wonder.
Every child goes through a lying phase. When your daughter tells the truth, praise her and never accuse her of lying unless you know for sure it is a lie. When she does lie, try to make the punishment fit the crime. Punishments should also be short term, especially at age 7.
Also, learn to listen to your child as well as talking to them. My children, son 10 and daughter 17, tell me everything (well, almost everything), because I listen to them and I don't judge. When they do something wrong, I let them know what they did wrong, why it was wrong, and what they should do the next time. If the crime is severe enough, I will try to make the punishment fit the crime or I will take away computer/video game/tv time.
Try to think about why your child is lying. Is she lying to get out of trouble and, if so, is it because your punishments are too harsh or too long term? My children do not like to be punished, but they are not afraid of punishment because I try to make the punishments fair and reasonable, so they are not afraid to tell me the truth.
If your daughter continues to lie and does not outgrow this phase, please consider counseling. My daughter had a serious problem with lying and I eventually had to put her into counseling (she was about 12 years old).