In a small saucepan over low heat, heat the condensed milk, chocolate, and butter until melted, smooth, and well combined. Pour chocolate mixture into the bowl of a stand mixer. Let the mixture cool until only slightly warm, or even room temperature. This should take about 5- 10 minutes.
While the chocolate cools, combine the biscuit mix and cornstarch in a bowl. Set aside.
Turn mixer on a low speed, this will finish cooling the chocolate mixture. Once it is no longer hot, add the egg and vanilla. Beat until just combined.
Scrape the bowl down and add the dry ingredients. Beat on low until just combined. Add 6 oz of peanut butter chips and mix until just combined. Cover bowl with plastic wrap and chill in the fridge for 2 hours, or even overnight.
When ready to bake, preheat the oven to 350℉ and line baking sheets with parchment paper or silicone baking mats.
Use a small cookie scoop (or a heaping tablespoon of dough) to form balls and drop them 2 inches apart on the prepared baking sheets. Top the cookie balls with extra peanut butter chips.
Bake 10- 13 minutes. Let them cool on the baking sheets for about 5 minutes before removing to a cooling rack.