Moist fluffy cupcakes are all about adding volume to your batter when mixing. This means that knowing how to mix your batter is important. Over mixing the batter can collapse cupcakes, thus taking away all that fluffiness. As a rule of thumb, cream butter and sugar until the sugar has almost melted - 2 minutes. Add eggs one at a time - and make sure each one is well incorporated. Then, add the flour and liquid - but mix until just incorporated. It is easy to over mix at this stage.