In a liquid measuring cup, whisk together the melted coconut oil, cocoa powder, and 3 tablespoons maple syrup. Taste it, and add more maple syrup if you want it sweeter.
Pour a small amount of the chocolate mixture into the bottom of each liner or mold cavity, until the bottom is thinly covered. Don't use more than half the chocolate mixture for this. Place the liners in the refrigerator for about 10 minutes to harden.
Once the bottom layer of chocolate is hard, place about 1/2 teaspoon of peanut butter over the chocolate. Pour enough of the chocolate mixture over the peanut butter to cover it completely (if at any point the remaining chocolate gets too thick, warm it in the microwave for 10-15 seconds to remelt). Repeat until all are covered. If desired, sprinkle the top with flaky sea salt. Return to the refrigerator to harden completely.
Store at cool room temperature, or in the refrigerator if you live in a warm climate.
Honey can be substituted for the maple syrup in equal amounts.