Place the crushed biscuits in a mixing bowl and pour in the melted butter. Mix until the crumbs are evenly coated and resemble wet sand. Press this mixture firmly into the base of a buttered springform pan, creating an even layer. Bake at 180°C (350°F) for 10 minutes until lightly golden, then set aside to cool.
Heat the heavy cream in a saucepan over just simmering heat until you see tiny bubbles around the edges. Pour the hot cream over the chopped dark chocolate in a mixing bowl and let sit for 2 minutes to soften. Then, gently stir with a spatula until smooth, glossy, and velvety — the ganache should be thick but pourable.
Pour the ganache over the cooled crust, spreading it evenly with a spatula. Allow it to set at room temperature for about 15 minutes, then transfer to the fridge and chill for at least 4 hours or overnight until firm and glossy.
Once the ganache is set, spread a generous layer of peanut butter on top using a spatula, creating a smooth, even surface. Return the tart to the fridge for about 30 minutes to help the peanut butter firm up slightly.
Just before serving, sprinkle flaky sea salt over the top for a salty crunch, and add chopped peanuts if desired for extra texture. Use a hot knife dipped in warm water to slice cleanly through the tart, revealing the rich layers inside.