Preheat your oven to 350°F (175°C). In a bowl, combine crushed chocolate graham crackers, melted butter, and sugar until fully blended. Press the mixture firmly into the bottom and up the sides of a 10-inch tart pan. Bake for 10 minutes until lightly golden, then let cool completely.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Pour over chopped dark chocolate in a heatproof bowl and let sit for 5 minutes, then stir until smooth. Cool slightly before whisking in the eggs, vanilla extract, and salt.

Pour the chocolate filling into the cooled crust and spread evenly. Bake for about 20 minutes until the edges are set but the center has a slight wobble. Let cool at room temperature for at least an hour, then refrigerate until chilled.

In a bowl, dissolve gelatin and espresso powder in warm water. Let cool slightly. In another bowl, whip cold heavy cream and powdered sugar until soft peaks form. Gradually add gelatin mixture, continue whipping until stiff peaks form.

Using a piping bag fitted with a large star tip, pipe rosettes of espresso whipped cream on top. Chill for at least 30 minutes before serving.

Gently slice the tart into wedges and serve chilled. Optionally garnish with fresh berries or cocoa powder.