Prepare diet chocolate and chopped nuts.
Melt the coconut oil.
In a bowl, add the diet chocolate, coconut oil, and unsweetened cocoa powder, then melt them together using a double boiler.
Add Greek yogurt and sweetener, then mix well.
Add the nuts and mix evenly.
Line a baking pan with parchment paper and pour in the batter.
Chill in the refrigerator for at least 2 hours, then cut into desired pieces.