Line 12 standard muffin cups with paper liners.

In a medium microwave-safe bowl, heat chocolate chips and shortening on Medium (50%) 1 minute. Stir, then continue to microwave on Medium in 30-second intervals, stirring after each, until mixture is smooth.

Spoon 1 generous tablespoon melted chocolate into bottom of each paper liner. Use a small clean brush or pastry brush to gently brush some of the chocolate partially up the sides of the cups. Freeze cups 10 minutes until chocolate is set.

Spread 1 teaspoon peanut butter into bottom of each cup. Return to freezer; freeze 10 minutes until peanut butter is set.

Spread 1 teaspoon marshmallow creme on top of peanut butter layer (it may not spread as well right away, but will spread on its own over time). Freeze 5 minutes.

Top each cup with remaining melted chocolate, about 1 to 1 1/2 tablespoons each; smooth tops as much as possible. Top with flaky sea salt, if desired.

Freeze 30 minutes or until chocolate is fully set.

Serve cold or at room temperature. Store covered in refrigerator up to 5 days.