In a small saucepan, bring the fresh cream to a boil over medium heat, stirring occasionally. Do not let it boil too much, just enough to reach boiling point. Melt the chocolate
Meanwhile, break the dark chocolate into small pieces and place in a heatproof bowl. Once the cream is hot, pour it over the chopped chocolate and let it sit for a minute. This will allow the chocolate to begin to melt.
Mix the ganache
After a minute, mix gently with a spatula or whisk until the chocolate is completely melted and the cream becomes smooth and homogeneous.

Add butter (optional)
If you want a softer and shinier ganache, add the butter in small pieces to the ganache and stir until it has completely melted. This step will make the ganache silkier and perfect for decorating. Add powdered sugar (optional)
If you prefer a sweeter ganache, you can add 1 or 2 tablespoons of powdered sugar to taste, stirring well to dissolve it into the cream.
Cool and use
Let the ganache cool to room temperature before using it as a cake filling or topping. If you want it thicker, you can put it in the fridge for a while. If you prefer it softer, just leave it at room temperature to soften it.