FOR THE FILLING
1/2 cup heavy cream
4 ounces semisweet or bittersweet chocolate, very finely chopped, plus optional chocolate shavings, for garnish
2 tablespoons unsalted butter, at room temperature and cut into small pieces
Lightly sweetened whipped cream or crème fraîche, for garnish
For the tart shells: Preheat the oven to 350 degrees. Grease the tartlet pans with cooking oil spray.
Combine the crumbs, sugar and salt in a mixing bowl. Pour the butter over the mixture and stir with a fork until all the crumbs are moistened. Divide the mixture evenly among the pans; use your fingertips to firmly press it along the bottom and up the sides. Freeze the pans for 10 minutes.
Place the tart shell pans on a baking sheet; bake (middle rack) for about 10 minutes or until the crusts take on some color. If you plan to assemble the tarts right away, cool the shells to room temperature, then freeze for 10 minutes before unmolding very carefully – these are fragile – and filling. If you are not assembling right away, store at room temperature in an airtight container for up to 2 days, then freeze for 10 minutes and unmold before filling (see headnote).
For the filling: Combine the cream and chocolate in a small saucepan over low heat. Cook for about 5 minutes, stirring constantly, until the chocolate has melted and the mixture is velvety smooth. Be careful; you don’t want to give this too much heat and you certainly don’t want it to come to a boil. Turn off the heat, keep the pan on the burner and add the butter a little at a time, stirring the small pieces in until they melt. Once again, the mixture will be smooth and, this time, a little shiny (a ganache). Transfer the ganache to a liquid measuring cup with a spout. The yield is a generous 3/4 cup.
Place the tartlet shells on a tray. (That makes it easy to move them around.) Fill them with the ganache, dividing it evenly among the crusts and pouring it as full to the rim as possible. Use an offset spatula to help spread the filling as needed. If you have any ganache left over, save it: You can reheat it gently in a microwave and then pour it over ice cream.
Loosely cover the tray with aluminum foil and carefully slide it into the refrigerator; chill until the ganache is cold and firm, about 2 hours, or for up to 1 day.
At serving time, top the tartlets with whipped cream or crème fraîche and scatter over some chocolate shavings, if using. Serve right away.