Line a 12-cup muffin tin with paper liners. Set aside.

In a medium bowl, beat together the peanut butter, powdered sugar, softened butter, and fine sea salt with a rubber spatula until completely smooth and thick, about 2 minutes. Fold in the mini chocolate chips. The mixture should hold its shape when pressed.

Divide the peanut butter mixture into 12 equal portions (about 1.5 tablespoons each). Roll into balls, then flatten into disks slightly smaller than the muffin liners. Place on a plate and refrigerate while you prepare the chocolate.

Place 1.5 cups of the chocolate chips and coconut oil in a microwave-safe bowl. Microwave in 30-second bursts, stirring between each, until completely melted and smooth, about 90 seconds total.

Spoon about 1 tablespoon of melted chocolate into each prepared liner. Use the back of the spoon to spread the chocolate up the sides, creating a thin even shell that covers the bottom and about 0.5 inch up the sides. Freeze for 10 minutes until set.

Place one peanut butter disk into each chocolate shell, pressing gently so it sits level without touching the sides.

Melt the remaining 0.5 cup chocolate chips. Spoon about 1 tablespoon over each cup to cover the filling completely, spreading to the edges to seal. Tap the pan gently on the counter to smooth the tops. Sprinkle with flaky salt if desired.

Refrigerate until completely firm, about 30 minutes. Peel off paper liners before serving. Store refrigerated in an airtight container for up to 2 weeks.