Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate graham crackers, melted unsalted butter, and sugar until resembling wet sand. Press into the bottom and sides of a 10-inch tart pan. Bake for 10 minutes and cool completely.

In a saucepan, bring 1 1/4 cups of heavy cream to a gentle boil. Pour over chopped bittersweet or dark chocolate in a bowl. Let sit for 5 minutes, stir until smooth. Allow chocolate ganache to cool slightly before whisking in eggs, vanilla extract, and salt.

Pour the chocolate filling into the cooled tart crust. Bake for about 20 minutes until edges are set and center is wobbly. Cool completely on a wire rack.

Dissolve gelatin and espresso powder in 2 tablespoons of warm water. In a mixing bowl, combine cold heavy cream and powdered sugar; whip until soft peaks form. Gradually add gelatin mixture while whipping until stiff peaks form.

Pipe espresso whipped cream over the cooled tart. Serve with additional whipped cream or sprinkle with cocoa powder.

Cover and refrigerate for at least 1 hour before serving.