Line a 12 cup muffin tin with paper or silicone liners.
Place 1 cup (180g) chocolate chips in a microwave safe bowl. Microwave in 20 second intervals, stirring after each, until fully melted and smooth, about 1 to 2 minutes total.
Spoon 1 teaspoon of melted chocolate into the bottom of each liner. Use the back of the spoon to spread it slightly up the sides. Chill for 5 minutes until set.
In a medium bowl, mix peanut butter, powdered sugar, vanilla, and salt until smooth. Fold in pretzels or extra chips if using.
Divide the peanut butter mixture evenly among the cups, about 1 tablespoon or 20g per cup. Press gently to flatten.
Melt the remaining ½ cup (90g) chocolate chips. Spoon over the peanut butter layer to cover completely.
Sprinkle with sea salt if desired. Chill for at least 30 minutes until firm.