Break the chocolate into pieces and place it into a saucepan with the condensed milk. Place over low heat on the stove and stir gently until the chocolate has fully melted.
Pour the chocolate fudge mixture over the cookie dough in the tin and spread it out evenly. Work quickly as it'll start setting as soon as you remove it from the heat.
Divide the remaining cookie dough into 4 pieces and use your hands to flatten each piece as much as you can. Place the pieces over the fudge then use a toothpick to swirl them together. (see video if you're unsure)
Top the cookie with extra mini peanut butter cups and chocolate chips.
Bake for 22-25 minutes, until golden brown all over and slightly pulling away from the edges. (note 3)
Leave it in the tin to cool completely then pop it in the fridge for 2-3 hours to set.
Once set, remove the cookie from the tin, cut it into slices and enjoy!