Simple Spread

• The most straightforward way to frost a cookie with only a knife is simply to spread a layer of your frosting across the surface. Scoop up enough frosting with the knife to cover the cookie without pulling up excess crumbs, hold the cookie level in one hand and pull the frosting over the top. Once finished, use the knife to scrape away any frosting that went over the edge of the cookie. Many kitchen knives have a flat edge and a slightly serrated edge. Use the flat edge to smooth out any lines unless you want them.

Dip and Finish

• Rather than spread the frosting across the cookie, you can dip the cookie into the icing and smooth out any rough spots and edges with the knife. This technique works especially well with thinner frostings, such as royal icing. Hold the cookie steady and flat, dip it into the icing then twist slightly as you pull up and away. Go over the top of the cookie with the flat, smooth edge of the knife to even it out and run the knife along the edges to catch any over-spill. Check the bowl of frosting between each dip and remove any crumbs that were left behind.

Disguising Imperfections

• If you simply cannot get the frosting as smooth as you'd like with just a knife, consider using a technique to disguise them. Swirl the tip or the end of the handle of the knife around the frosting to create a homey, freshly decorated look. Add sprinkles, small candies or other edible decorations to the top of the cookie to distract from any unwanted lines. Pull the knife over the cookie, alternately pushing down and pulling up to create waves of frosting.

Dress It Up

• Using a knife rather than a piping bag doesn't restrict you to simple, flat swaths of monotone frosting for your cookies. Consider spreading stripes of two or three different colors of frosting over the top of your cookies, then dragging the tip of your knife through the colors perpendicularly to the lines to create a swag look. Alternatively, spread a single layer of frosting over the cookies then drop spots of another color frosting on top with the tip of the knife. Create a base with icing, allow it to dry for several hours, then spread a layer of thinned-out frosting in a different hue over the base for sheer, two-toned look.