Crush the graham crackers into fine crumbs using a food processor or by placing them in a plastic bag and crushing with a rolling pin. Mix the crumbs with melted butter until evenly coated and crumbly.
Press the graham cracker mixture evenly into the bottom and up the sides of a tart pan with a removable bottom. Use the back of a spoon or your fingers to smooth and compact the crust.
Bake the crust in a preheated oven at 180°C (350°F) for about 10 minutes, until fragrant and slightly golden. Remove from the oven and let cool slightly while you prepare the filling.
Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer, with small bubbles forming around the edges.
Pour the hot cream over the chopped dark chocolate in a bowl. Let sit for 2 minutes to allow the chocolate to soften.
Stir gently with a spatula until the mixture is smooth, glossy, and fully combined. Add the peanut butter and stir until silky and well incorporated.
Pour the chocolate peanut butter filling into the cooled crust, spreading it evenly with a spatula. Tap the pan gently on the counter to settle the filling.
Bake the tart in the oven at 180°C (350°F) for 15-20 minutes, until the edges are set but the center still has a slight jiggle.
Remove the tart from the oven and let it cool completely at room temperature. Then, transfer to the refrigerator and chill for at least 2 hours to allow the filling to set firmly.
Optional: Before serving, sprinkle a pinch of sea salt on top for contrast and extra flavor.
Slice the tart with a sharp knife, wiping the blade clean between cuts for neat slices. Serve at room temperature for the best texture and flavor.