Prep your pan: Line a 12-cup muffin tin with paper liners. This makes cleanup easy and helps the cups release cleanly.
Warm the wet ingredients: In a small saucepan over low heat, combine 1/2 cup coconut oil (or butter), 3/4 cup peanut butter, and 1/3 cup honey or maple syrup.
Stir until smooth and just warm, then remove from heat. Mix in 1 teaspoon vanilla and a pinch of salt.
Stir in the oats: Add 2 1/2 cups rolled oats to the warm mixture. Stir until every oat is coated.
If using add-ins like 2 tablespoons chia seeds or 1/4 cup mini chocolate chips, fold them in now.
Fill the cups: Divide the oat mixture evenly among the muffin liners, pressing it down firmly with the back of a spoon. Aim for compact, level bases so the chocolate sets smoothly.
Melt the chocolate: In a microwave-safe bowl, melt 1 cup chocolate chips with 1 tablespoon coconut oil in 20-second bursts, stirring between each. Or melt over a double boiler until glossy and smooth.
Add the topping: Spoon about 1 tablespoon melted chocolate over each cup, tilting the pan slightly to spread it to the edges.
Sprinkle with flaky sea salt if you like.
Chill to set: Refrigerate for 1–2 hours, or until the chocolate is firm and the cups hold together when lifted.
Serve and store: Peel away the liners and enjoy. Keep leftovers chilled for best texture.