Mix graham cracker crumbs with melted butter and salt in a bowl until the mixture resembles wet sand. Press this mixture firmly into the bottom of a 23cm tart pan to form an even crust. Bake at 180°C (350°F) for 8-10 minutes until golden and fragrant, then let it cool completely.
Place the chopped dark chocolate in a heatproof bowl. Heat the heavy cream until just simmering, either on the stovetop or in the microwave, then pour it over the chocolate. Let sit for 2 minutes, then stir gently until the ganache is smooth and glossy.
Pour the chocolate ganache into the cooled crust, spreading it out evenly with a spatula. Chill in the refrigerator for about 2 hours until set but still glossy.
Warm the peanut butter slightly in the microwave for about 15 seconds until it's runny and easy to swirl. Drop spoonfuls of peanut butter over the set chocolate filling.
Use a spatula or toothpick to gently swirl the peanut butter into the chocolate ganache, creating a marbled effect. Be careful not to disturb the crust or over-mix.
Sprinkle the chopped toasted peanuts over the top for crunch and aroma. Finish with a pinch of sea salt for added flavor.
Refrigerate the tart for at least 4 hours or overnight to allow the filling to fully set. Before serving, let it sit at room temperature for a few minutes for optimal flavor and texture.