Grease and line a 20cm square cake tin.
Make base: In a food processor, pulse all the base ingredients together with a pinch of salt, until crumbly and mealy. Press mixture into the base of the prepared tin, using the base of a cup measure to create an even layer. Set aside in the fridge.
Make peanut butter cheesecake: Clean out food processor, then add all peanut butter cheesecake ingredients and blitz together with a pinch of salt, until smooth and creamy. Pour over the base and spread out into an even layer, tapping on the bench a few times to create a level surface.
Make topping: Sprinkle the roasted peanuts over the top of the peanut butter cheesecake in a single, even layer. Place into the freezer to chill for about 1 ½-2 hours, until completely firm and set.
Melt chocolate in a heat-proof bowl in the microwave in 30 second increments, until completely melted. Drizzle melted chocolate over the peanuts and tilt the tin so that the chocolate spreads into an even layer. Sprinkle over flaky sea salt, if desired. Chill again for 15minutes, until chocolate is set.
Once set, use a large serrated knife to cut the slice into rectangles about 2cm x 5cm. Store in the fridge.