Adjust rack one-third up from the bottom of the oven, and preheat to 375 degrees. Separate the bottom from the sides of a 9 x 3 inch spring-form pan. Butter the sides only (if you butter the bottom the crust will stick to the bottom and it will be difficult to serve). Replace the bottom in the pan and set aside.

Crumble the cookies coarsely and place them in a food processor or a blender (or place them in a plastic bag and lb and roll them with a rolling pin) to make fine crumbs. You should have 2 cups of crumbs. Place them in a mixing bowl. Melt the butter and stir it into the crumbs until thoroughly distributed. Pour about two-thirds of the mixture into the prepared pan.

To form a thin layer of crumbs on the sides of the pan, tilt the pan at about a 45-degree angle and, with your fingertips, press a layer of the crumbs against the sides, pressing from the bottom up toward the top of the pan and rotating the pan gradually as you press on the crumbs. They should reach the top of the pan all the way around. Then place the pan upright on its bottom. Pour in the remaining crumbs and, with your fingertips, distribute them over the bottom of the pan to cover it. Press firmly to make a compact layer.

Bake for 7 to 8 minutes, remove from the oven, and cool completely.

In a heavy 2 quart saucepan, dissolve the coffee in the water. Add 1/2 cup sugar, and stir over moderate heat to dissolve. Adjust the heat to low, add the chocolate, and stir until melted and smooth. Remove from the heat and let stand for a few minutes to cool slightly. Add the egg yolks one at a time, stirring them in with a wire whisk. Set aside to cool completely.

In the large bowl of an electric mixer, whip the cream until it holds a shape but is not really stiff. Set aside.

In the small bowl of an electric mixer, with clean beaters, beat the egg whites until they are foamy. Add the salt and the cream of tartar and continue to beat until the whites hold a soft shape. Reduce the speed to moderate and gradually add the reserved 3/4 cup sugar, one large spoonful at a time. Beat briefly between additions. Then increase the speed to high again and beat for a few minutes until the meringue is quite firm, but not stiff or dry.

Gradually, in two or three additions, fold most of the chocolate into the whites, and then fold the whites into the remaining chocolate. In a very large mixing bowl fold together the whipped cream and the chocolate mixture.

Pour into the chocolate-cookie crumb crust, smooth the top or form a swirling pattern, and place in the freezer. After about an hour or so cover the top airtight with plastic wrap. Freeze overnight or for up to 2 weeks.

The mousse may be removed from the pan just before serving or days before. With a firm, sharp, heavy knife, cut around the sides of the crust, pressing the blade firmly against the pan as you cut. Then release and remove the sides of the pan. Now, use a firm (not flexible) metal spatula (either a wide one or a long narrow one): Insert the spatula gently and carefully under the crust and case it around to release the dessert completely from the bottom of the pan. The dessert will be firm and sturdy and easy to transfer. If you are serving it soon, place it on a large, flat dessert platter; if you are going to store it again in the freezer, place it on a large piece of plastic wrap and wrap airtight. In either case, return it to the freezer until serving time. It does not freeze too firm, and will cut beautifully and easily. Use a sharp, heavy knife.

OPTIONAL DECORATION: Just before serving, cover the top of the mousse with whipped cream and/or a layer of overlapping Chocolate Leaves , and surround the whole dessert with a generous ring of large Chocolate-Covered Strawberries .