Line two baking sheets with parchment paper and place one of them in the freezer to chill while you mix the filling.
In a medium saucepan over low heat, melt the butter with the peanut butter, stirring just until smooth and warm, 1–2 minutes. Remove from the heat and stir in the powdered sugar and sea salt until a thick, cohesive dough forms.
Fold in the crispy rice cereal until evenly distributed. Let the mixture stand for 5 minutes to firm up slightly so it scoops cleanly.
Using a 1-tablespoon scoop, portion the mixture onto the chilled baking sheet, then roll each portion between your palms to form smooth balls. Freeze until very firm, about 20 minutes.
Set a heatproof bowl over a small pot of barely simmering water (the bowl should not touch the water). Add the dark chocolate and coconut oil and stir gently until melted and glossy. Keep the water at the lowest simmer to hold the chocolate fluid.
Working with 6–8 centers at a time (keep the rest frozen), dip each ball into the melted chocolate using a fork. Tap the fork on the bowl’s edge to let excess chocolate drip off, then return the coated ball to the second parchment-lined sheet. If the chocolate thickens, rewarm it briefly over the steam.
Let the chocolate set at cool room temperature for 30–45 minutes or refrigerate for 10–15 minutes until the shells are firm. If you’d like a decorative finish, rewarm a spoonful of chocolate and drizzle over the set candies, then let set again.