Step 1

To make the crust: In a medium bowl, stir together all of the crust ingredients until thoroughly combined and the texture of wet sand. Alternatively, use a food processor to combine the cookies and salt; pulse until you have fine crumbs, then stream in the melted butter and process until the mixture looks damp and holds together when squeezed.

Step 2

Transfer the mixture to a 9" tart pan (preferably with a removable bottom), then use the bottom of a measuring cup or glass, plus the pads of your fingers, to press it evenly into the bottom and the sides. Place the tart pan in the freezer while you make the filling and topping, at least 1 hour. (This step can be done up to 3 days ahead; cover the tart pan and store in the freezer.)

Step 3

To make the filling: Set up a double boiler by filling a medium saucepan with 1" of water and bringing it to a simmer. In a large bowl that will nest in the pot above the water, whisk the eggs and sugar until well combined. Set the bowl over the simmering water and cook, whisking constantly, until the mixture reaches 160°F on a digital thermometer, 2 to 3 minutes; it should be thick enough to coat the back of a metal spoon. Remove the bowl from the heat, add the chocolate, vanilla, salt, and espresso powder, and whisk gently until the chocolate is completely melted. Set aside to cool to room temperature, for at least 30 minutes.

Step 4

Meanwhile, whip the cream to soft peaks in a large bowl or the bowl of a stand mixer fitted with the whisk attachment. Transfer to a separate small bowl or container, cover, and place in the refrigerator.

Step 5

Rinse out the mixing bowl, then use the flat beater attachment to beat the butter and cream cheese together on medium speed until light and fluffy, 3 to 4 minutes. Add the cooled chocolate mixture and beat until light in color and fully incorporated, 2 to 3 minutes. Scrape down the bowl and beat once more to ensure that all of butter is fully incorporated.

Step 6

Working by hand, use a whisk to gently stir in about a third of the chilled whipped cream; this will lighten up the mixture so that the rest of the whipped cream can be incorporated more easily. Add the remaining whipped cream and fold gently until no white streaks remain.

Step 7

Transfer the filling to the frozen tart crust, spread into an even layer, cover, and chill in the refrigerator until set, about 2 hours.

Step 8

To make the topping: When ready to serve, beat the cream cheese in the bowl of a stand mixer fit with the flat beater attachment until completely smooth. Switch to the whisk attachment, add the cream, and whip until bubbles form all over the surface, about 30 seconds. Sift together the confectioners’ sugar, Instant ClearJel, and salt. With the mixer on medium, gradually sift in the sugar mixture, then beat to soft peaks.

Step 9

Spread the whipped cream topping over the center of the chilled tart, leaving the edges bare. If you’d like, use a vegetable peeler to shave the chocolate bar over the topping to make curls. Keep the tart refrigerated until ready to serve, then cut into slices.

Step 10

Storage information: Store the tart, without the whipped cream topping, in the refrigerator, covered, for up to 4 days. Once topped, store in the refrigerator for up to 2 days.