Line a standard muffin pan with 12 paper liners. Using a double boiler or microwave, melt the milk chocolate and coconut oil until smooth. Spoon about 2 teaspoons of melted chocolate in each cup. Use a spoon or knife to spread chocolate about halfway up the sides of the liners. Pop into the refrigerator to harden.
Combine the softened butter, peanut butter, sugars, and vanilla extract in a bowl and mix until it's light and fluffy. Add the flour and salt, and mix until combined and the dough forms. Fold in the mini chocolate chips.
Scoop 12 equal amounts of the peanut butter cookie dough and roll them into balls. Take the chocolate cups out of the refrigerator. Place one ball of cookie dough in each of the chocolate cups.
Using your hands or the bottom of a glass, gently press down the cookie dough to flatten it into an even layer, touching the sides of the chocolate cup.
Re-melt the remaining milk chocolate. Once smooth again, spoon equal amounts of the melted chocolate onto the tops of each peanut butter cup. There should be just the right amount left to cover all 12 cups. Now that the cups are done, pop them in the refrigerator to set completely