If you want to thin out your buttercream, it's best to add a little bit of milk or cream instead of water. This will help keep the flavor and texture of the frosting consistent. You can also add flavor by using a flavored coffee creamer, a bit of extra vanilla extract, or half and half. All options are ideal for giving the buttercream that smooth texture you want. Just make sure you use a little at a time so you don't thin out the buttercream too much.