Cutting a 6-inch round cake can be a piece of cake...no pun intended. The easiest way to do this is with a knife that is long enough to go from the center of the cake to the outer edge. If your blade is shorter than the radius of the cake, this will not work. Decide how big you would like to cut your slices, 1-inch thick slices work quite well. You can mark this roughly on the outer icing by scoring a little mark with your knife. Place the blade's tip in the center of the cake and gently cut downwards until the cut is done. The second cut will be an inch from the first cut and should be done exactly the same. After the initial slice, you will only need one cut per slice. The first slice will always need two cuts.