Place the coconut oil in a medium microwave-safe bowl. If it's solid, microwave it to melt for 20-30 seconds.
Use a hand whisk to whisk in the cocoa powder, vanilla, and stevia.
If your nuts are coarsely chopped, place them in a resealable bag and go over them back and forth with a rolling pin to create a finer chop.
Use a rubber spatula to mix the chopped nuts into the chocolate mixture.
Using a teaspoon, transfer the mixture to a silicone cordial mold, which you can get on Amazon.
Freeze for 20 minutes, then release from the mold. Store in the fridge, in an airtight container lined with wax paper, for up to a week.
Enjoy this chocolate straight out of the fridge - it's melty. It does set better the next day, so consider making it a day ahead.
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Notes
This chocolate is very melty because of the coconut oil. If this is an issue, you can use food-grade cocoa butter instead (it's available on Amazon and at Whole Foods). Alternatively, make it the day before you plan on enjoying it. Its texture improves after a full night in the fridge.