1(11.5 oz) bag milk chocolate chips (or semi-sweet, if you prefer)
Line a 24-cup mini muffin tin with paper liners. Set aside.
Mix together peanut butter, sugar, vanilla, and salt in a small bowl until smooth and well-combined. Taste and adjust sugar and salt as necessary. Set bowl in fridge to chill.
In a microwave-safe bowl, melt the chocolate chips in 15-second bursts until smooth and shiny. Drop teaspoonfuls of the melted chocolate evenly into prepared mini muffin cups, reserving the remaining chocolate. Using a small spoon or small silicone pastry brush, push the melted chocolate up the sides of each mini muffin cup. Place muffin tin in fridge until chocolate is firm, up to 30 minutes.
Once chocolate is firm, remove muffin tin from fridge. Remove peanut butter mixture from fridge. Top chocolate cups with teaspoonfuls of the peanut butter mixture, pressing mixture down with your finger to smooth out. Top peanut butter with teaspoonfuls of the remaining melted chocolate. Return muffin tin to fridge and chill until peanut butter cups are set. Peanut butter cups can be stored in the fridge for up to 5 days.