What is the best kind of icing to use on cookie cake? Im making a cookie cake for my daughters birthday, there will be lots of icing on the cake.

I like to use a pretty standard crusting vanilla buttercream for mine. It goes well.

Sure thing...I got it originally from the rose cake tutorial site (i am baker) and I love it! I'll give you the original and then my tweaks underneath. I do still use the original from time to time though. HTH

THE BEST BUTTERCREAM

2 cups shortening
8 cups confectioner's sugar
1/2 teaspoon salt
1 tablespoon clear imitation vanilla extract
6 fluid ounces heavy cream

In a large bowl, cream shortening with mixer until fluffy. Add confectioners sugar (2 cups at a time) until well blended. If the frosting gets to thick at any time you can add some of your heavy cream.
Add salt, vanilla, and heavy cream and blend until combined, then blend on high until frosting is light and fluffy. If you need to add additional cream you can. (up to 2 tablespoons)

**My differences depending on what I'm using it for is that I add about 1 Tbsp of powdered dream whip into the shortening after it's fluffy and before adding the confectioner's sugar and instead of 1 Tbsp vanilla, I use 1 1/2 tsp vanilla and 1 1/2 tsp creme bouquet.