1¾cupsgraham cracker crumbs (from about 8 ounces or 14 whole crackers)
1tablespooninstant espresso powder
12ouncesbittersweet chocolate, chopped
2tablespoonsunsalted butter, softened
Flaky sea salt, for sprinkling
In a small saucepan, melt the butter with the sugar and salt over medium-low heat, stirring, until the sugar dissolves, about 4 minutes.
In a medium bowl, mix the graham cracker crumbs with the butter mixture until the crumbs are evenly moistened. Press the crumbs evenly over the bottom and up the side of a 9-inch tart pan with removeable bottom. Refrigerate the crust until firm and well chilled, 30 minutes to 1 hour. Alternately, freeze the tart shell for 10 to 15 minutes, until firm.
In a medium saucepan, bring the cream and espresso powder just to a simmer. Remove from the heat and add the chocolate, butter and salt. Let stand for 2 minutes, then stir gently until smooth and glossy. Scrape the ganache into the tart shell and, using an offset spatula, smooth the top.
Refrigerate the tart until the surface is just starting to set, 15 to 20 minutes. Sprinkle the top with flaky sea salt, then refrigerate until the tart is firm, about 1 hour more.
Carefully remove the tart from the ring and transfer to a cake plate or platter. cut the tart into wedges and serve.
DO AHEAD: The tart can be refrigerated in an airtight container or covered with plastic for up to 1 week. Alternately, the tart can be wrapped tightly in plastic and frozen for up to 1 month.SERVING SUGGESTION: I use 1 1/4 cups of cream in the ganache, which means you're likely to have some leftover. This is perfect! Whip the cream with just a bit of sugar and some vanilla, and serve big puffy dollops of it over the finished tart.