@Oeconomist I imagine she was making chocolate cupcakes as chocolate is the go-to for valentines day and if you are trying to make anything with chocolate, typically people will rough chop the chocolate to make it melt faster and even if it's just a mix-in for cupcakes or something, giving the chocolate a bit of a chop can make the dessert have a bit of a rustic/handmade feel to it.
then take into account her cupcakes had something on top so there could have been any kind of nut or dried/fresh fruit, there are lots of things to chop for cupcakes.
also unsurprisingly if you aren't used to using a knife in the kitchen you can make a lot of small, dumb mistakes, some people are even intimidated by using a knife in the kitchen and mishandle them because of this.
also. after rereading the chapter, even though they somewhat appear to be cupcakes, not a single character refers to them as such, so they could be just about anything involving chocolate.