Birthday Cake, step 3

    

As you can see, the crumb coat is not thick at all, you should be able to see the cake through it, at least if it's chocolate!

A crumb coat is a very important step if you want a professional, clean looking cake.  It is very easy, but a bit time consuming.  The idea is that it will trap all the cake crumbs in so they don't get into the outer, thick layer of frosting.  Also, it smoothes out the cake, so you don't have to worry about that when you are applying the outer layer of icing.

Once you have filled and stacked the cake layers, put the cake in the fridge for about 30 minutes, until it is nice and chilled.  Then use some of the plain buttercream that you made to make the ring on the layers (in step 2) to thinly coat the outside of the cake.  First coat the top, then the sides.  I find the easiest way to do this is using a large offset spatula.  For the sides, I dip the spatula into the icing and spread it onto the cake, rather than putting a glob on the cake and then spreading it out.

Don't worry if you get some crumbs in this layer of icing, they won't show later.  Try to use this layer of icing as a chance to smooth out the surface of the cake.  For example, fill in gaps between the layers, so the sides are straight.

Put the cake back in the fridge and let it chill for, I'd say, about an hour minimum.  I let it chill overnight, because I had the time, but that isn't necessary.  Cut off/smooth out any spots where the frosting is sticking out.  Now you are ready for the thicker, outer layer of icing.  To be continued...