Finely chop the dark chocolate and unsweetened cocoa powder.
Melt the dark chocolate using a microwave or double boiler.
Add unsweetened cocoa powder, Greek yogurt, milk, and stevia to the melted chocolate and mix thoroughly.
Line a square container with parchment paper and pour in the chocolate mixture.
Smooth the surface and let it set in the refrigerator for 2-3 hours.
Cut the set chocolate into small cubes and coat evenly with cocoa powder.