Preheat the oven to 180C and line several baking sheets with parchment paper; set aside.
In a large bowl, use an electric whisk to combine the brown sugar, peanut butter, butter & granulated sugar until smooth, about 4 minutes. Add the egg to the mixture & beat until incorporated, being sure to scrape down the sides. Add the oil and vanilla extract, and combine fully.
In a different bowl, whisk together the flour, bicarbonate and baking powder. Pour the flour mix into the butter mixture and beat with the mixer on slow until everything is just combined. Use a spatula to add the peanuts into the mix and make sure you’ve scraped down the sides.
Roll the dough into small balls (weighing 14g each, which looks small but is actually the perfect size!) and gently press them into small disks before placing on the prepared baking sheet, leaving about 2 inches between each. Repeat with the remaining dough**.
Using the tines of a fork, gently press across the tops of the cookies to create a design. The cookies can be kept in the fridge while waiting for the oven to preheat.
Bake the sheet of cookies for 7-9 minutes, or until they are just ever so slightly beginning to brown. The cookies will be extremely soft when removed from the oven, so leave to cool on the sheet for a few minutes before moving to a wire rack to cool completely.
In a smallish bowl, combine the chocolate and the butter. In bursts of 20 seconds, gradually melt the two together in the microwave. Be very careful to take it out after each 20 seconds and to stir thoroughly, as this can easily burn.
When the cookies are completely cooled, gently dip one side of each into the bowl of melted chocolate. Place the half-dipped cookie onto a wire rack to dry and sprinkle the chocolate with a small pinch of coarse sea salt***. Enjoy!