Take your tube of cookie dough out of the refrigerator and let it sit on the counter for about 30 minutes. You don't HAVE to do this but it makes the process easier if the dough isn't quite so cold. You'll need a 10-inch fluted tart pan or a springform pan. Slice the cookie dough open all the way across the top with a sharp knife so you can take out the whole roll of dough in one piece. Place it in the center of your tart pan. If you want to keep your hands clean, place a piece of plastic wrap about the size of the pan over the dough. Now start to squish the dough toward the outsides of the pan. You're flattening it out so it spreads across the whole bottom of the pan, making a crust. Pretend you're back in kindergarten playing with Play-Doh. Squish it to the edges and up the sides. You'll have just enough dough to make a rim. Remove the plastic wrap and make your final squishes, checking to see if you have the dough pretty evenly squished so it's not too thin in any parts and there are no holes.
Bake the cookie dough according to the directions on the package: 350 degrees for about 12 minutes should do it. You basically want a lightly golden, giant cookie.
Remove the pan from the oven and get out a regular teaspoon. Gently run the back of that teaspoon around the inner rim of the dough along the bottom of the pan, just lightly squishing the dough down so the tart rim becomes a little better defined. Then let your cookie start to cool. While the tart is still slightly warm, carefully push it out from the pan and place it on a serving platter.
While the cookie is baking or cooling, you can start your filling. You are making a French ganache--a very simple version. Chop the chocolate finely; the size of small dice is fine. Put this chocolate in a heatproof glass bowl. Heat your cream in a small saucepan just to the boiling point. This isn't much cream, so keep an eye on it. When it starts to bubble around the edges, you are getting very close. You don't want it to boil over, but you want the cream to be very hot. Pour the cream over your chocolate and immediately take a whisk and stir. The hot cream is intended to melt the chocolate. Keep stirring until all of the chocolate is melted and you have a glossy, smooth mixture. If your cream cools too quickly or your chocolate chunks are too large, they won't all melt. If this happens you can put the bowl in your microwave and heat on Medium High for about 15 seconds. Don't do it very long or you'll burn your chocolate. Try not to use the microwave at all unless absolutely necessary.
Even if your cookie is not completely cool yet, you can proceed. Pour your filling into the cookie crust. Spread it evenly across the bottom of the tart. It will only be about a quarter of an inch thick, maybe a little less. Don't worry. This is one rich dessert!
Now, you have two options. If you want your tart to stay bright and glossy, let it sit somewhere on the counter to "firm up." This will take 5 to 6 hours at room temperature. Keep it out of the sun. When you serve it, the texture will be creamy and wonderful.
Option two is to put your tart in the refrigerator. When you do this, the chocolate will firm up more like a candy bar but you'll be amazed at how rich the candy bar taste will be. The glossiness goes away with the cold, however. Another advantage to the colder version is, if you are serving this on a buffet, when the tart is cold, you can slice it into really narrow slices and they will hold together perfectly. This allows you to have little narrow pieces that people can pick up and just pop into their mouths.
Anytime you have it stored in your refrigerator, you can always take it out ahead of time (a couple of hours) and let it soften up. Be sure to store the leftovers in the refrigerator to keep it fresh. It will last for several days in the refrigerator and you can just keep nibbling at it.
Try it plain the first time and then you judge whether you want to doctor it up. With just three ingredients, you'll have a beautiful dessert that qualifies for the most fancy of dinner parties. Check out the picture. You'll see what I mean.