Put dark chocolate and heavy cream in a bowl and melt using a double boiler or microwave.
Mix the melted mixture thoroughly and let it cool at room temperature for a while.
Line a square mold with parchment paper and pour the mixture, spreading it evenly.
Chill in the refrigerator for at least 2 hours until set.
Remove the hardened chocolate and cut into desired sizes.
Dust evenly with cocoa powder using a sieve.