Line a 9 x 9 inch square baking tin with parchment paper.
In a large bowl, combine the cookie crumbs, the soft brown sugar, and the cooking salt.
Add the cooled melted butter, 1 ¼ cups of creamy peanut butter, and the vanilla extract to the dry ingredients, then mix until everything is combined.
Pour the wet biscuit base into the prepared baking tin and smooth out the top. Place this in the fridge for 30 minutes to harden.
In a microwave-safe bowl, combine the milk chocolate and the remaining ¼ cup of peanut butter. Microwave the bowl in 15-second intervals, stirring in between each break until the chocolate is completely melted and combined with the peanut butter.
Remove the biscuit base from the fridge and pour the chocolate on top. Smooth it out and then sprinkle the teaspoon of flakey sea salt on top. Place this back into the fridge for at least 1 hour before removing it from the baking tin, slicing, and serving.