I don't know if you've ever frozen whipped cream, but I have and it comes out weird. It also melts really quickly We don't want either of these things to happen.
So, we're making some stabilized whipped cream using gelatin. Veggie friends out there, agar-agar works just as well. Other methods involve cornstarch and Whip It, but I'm using gelatin.
You will need:
1 tsp unflavored gelatin
1 Tbs cold water
1 c cold heavy or whipping cream
3 Tbs sugar
1 tsp vanilla
Sprinkle the gelatin over the cold water, and let it sit for a minute or so until it's absorbed. Then, microwave the gelatin at high heat for 30 seconds so it can "bloom." It should finish clear, but not melted.
In the now-cold mixing bowl, beat the cold cream with a cold whisk (sensing a theme here?) until it gets thick and starts to form peaks.
Gradually add in your sugar. Confectioners sugar is best, but granulated sugar works fine, too.
While the mixer is still running, slowly pour in your gelatin mixer until it's incorporated. Add vanilla, and add whatever other flavors you'd like.
To get that authentic Carvel look, separate a bit of whipped cream, and fold in blue food coloring until you reach your desired hue.
Put all of these things in the refrigerator until you're ready to decorate.